Implement use-after-free detection using junk and stash.

On deallocation, sampled pointers (specially aligned) get junked and stashed
into tcache (to prevent immediate reuse).  The expected behavior is to have
read-after-free corrupted and stopped by the junk-filling, while
write-after-free is checked when flushing the stashed pointers.

Makefile.in

@@ -284,6 +284,7 @@ TESTS_UNIT := \
 	$(srcroot)test/unit/thread_event.c \
 	$(srcroot)test/unit/ticker.c \
 	$(srcroot)test/unit/tsd.c \
+	$(srcroot)test/unit/uaf.c \
 	$(srcroot)test/unit/witness.c \
 	$(srcroot)test/unit/zero.c \
 	$(srcroot)test/unit/zero_realloc_abort.c \

configure.ac

@@ -1564,6 +1564,23 @@ if test "x$enable_opt_size_checks" = "x1" ; then
 fi
 AC_SUBST([enable_opt_size_checks])
 
+dnl Do not check for use-after-free by default.
+AC_ARG_ENABLE([uaf-detection],
+  [AS_HELP_STRING([--enable-uaf-detection],
+  [Allow sampled junk-filling on deallocation to detect use-after-free])],
+[if test "x$enable_uaf_detection" = "xno" ; then
+  enable_uaf_detection="0"
+else
+  enable_uaf_detection="1"
+fi
+],
+[enable_uaf_detection="0"]
+)
+if test "x$enable_uaf_detection" = "x1" ; then
+  AC_DEFINE([JEMALLOC_UAF_DETECTION], [ ])
+fi
+AC_SUBST([enable_uaf_detection])
+
 JE_COMPILABLE([a program using __builtin_unreachable], [
 void foo (void) {
   __builtin_unreachable();

include/jemalloc/internal/cache_bin.h

@@ -98,7 +98,7 @@ struct cache_bin_s {
 	 * when the array is nonempty -- this is in the array).
 	 *
 	 * Recall that since the stack grows down, this is the lowest address in
-	 * the array.
+	 * the array.  Only adjusted when stashing items.
 	 */
 	uint16_t low_bits_full;
 
@@ -107,7 +107,7 @@ struct cache_bin_s {
 	 * is empty.
 	 *
 	 * The stack grows down -- this is one past the highest address in the
-	 * array.
+	 * array.  Immutable after initialization.
 	 */
 	uint16_t low_bits_empty;
 };
@@ -136,6 +136,26 @@ cache_bin_array_descriptor_init(cache_bin_array_descriptor_t *descriptor,
 	descriptor->bins = bins;
 }
 
+JEMALLOC_ALWAYS_INLINE bool
+cache_bin_nonfast_aligned(const void *ptr) {
+	if (!config_uaf_detection) {
+		return false;
+	}
+	/*
+	 * Currently we use alignment to decide which pointer to junk & stash on
+	 * dealloc (for catching use-after-free).  In some common cases a
+	 * page-aligned check is needed already (sdalloc w/ config_prof), so we
+	 * are getting it more or less for free -- no added instructions on
+	 * free_fastpath.
+	 *
+	 * Another way of deciding which pointer to sample, is adding another
+	 * thread_event to pick one every N bytes.  That also adds no cost on
+	 * the fastpath, however it will tend to pick large allocations which is
+	 * not the desired behavior.
+	 */
+	return ((uintptr_t)ptr & san_cache_bin_nonfast_mask) == 0;
+}
+
 /* Returns ncached_max: Upper limit on ncached. */
 static inline cache_bin_sz_t
 cache_bin_info_ncached_max(cache_bin_info_t *info) {
@@ -232,6 +252,20 @@ cache_bin_empty_position_get(cache_bin_t *bin) {
 	return ret;
 }
 
+/*
+ * Internal.
+ *
+ * A pointer to the position with the lowest address of the backing array.
+ */
+static inline void **
+cache_bin_full_position_get(cache_bin_t *bin, cache_bin_info_t *info) {
+	cache_bin_sz_t ncached_max = cache_bin_info_ncached_max(info);
+	void **ret = cache_bin_empty_position_get(bin) - ncached_max;
+	assert(ret <= bin->stack_head);
+
+	return ret;
+}
+
 /*
  * As the name implies.  This is important since it's not correct to try to
  * batch fill a nonempty cache bin.
@@ -359,13 +393,17 @@ cache_bin_alloc_batch(cache_bin_t *bin, size_t num, void **out) {
 	return n;
 }
 
+JEMALLOC_ALWAYS_INLINE bool
+cache_bin_full(cache_bin_t *bin) {
+	return ((uint16_t)(uintptr_t)bin->stack_head == bin->low_bits_full);
+}
+
 /*
  * Free an object into the given bin.  Fails only if the bin is full.
  */
 JEMALLOC_ALWAYS_INLINE bool
 cache_bin_dalloc_easy(cache_bin_t *bin, void *ptr) {
-	uint16_t low_bits = (uint16_t)(uintptr_t)bin->stack_head;
+	if (unlikely(cache_bin_full(bin))) {
-	if (unlikely(low_bits == bin->low_bits_full)) {
 		return false;
 	}
 
@@ -377,7 +415,39 @@ cache_bin_dalloc_easy(cache_bin_t *bin, void *ptr) {
 	return true;
 }
 
-/**
+/* Returns false if failed to stash (i.e. bin is full). */
+JEMALLOC_ALWAYS_INLINE bool
+cache_bin_stash(cache_bin_t *bin, void *ptr) {
+	if (cache_bin_full(bin)) {
+		return false;
+	}
+
+	/* Stash at the full position, in the [full, head) range. */
+	uint16_t low_bits_head = (uint16_t)(uintptr_t)bin->stack_head;
+	/* Wraparound handled as well. */
+	uint16_t diff = cache_bin_diff(bin, bin->low_bits_full, low_bits_head);
+	*(void **)((uintptr_t)bin->stack_head - diff) = ptr;
+
+	assert(!cache_bin_full(bin));
+	bin->low_bits_full += sizeof(void *);
+	cache_bin_assert_earlier(bin, bin->low_bits_full, low_bits_head);
+
+	return true;
+}
+
+JEMALLOC_ALWAYS_INLINE cache_bin_sz_t
+cache_bin_nstashed_get(cache_bin_t *bin, cache_bin_info_t *info) {
+	cache_bin_sz_t ncached_max = cache_bin_info_ncached_max(info);
+	void **full = cache_bin_full_position_get(bin, info);
+
+	uint16_t nstashed = cache_bin_diff(bin, (uint16_t)(uintptr_t)full,
+	    bin->low_bits_full) / sizeof(void *);
+	assert(nstashed <= ncached_max);
+
+	return nstashed;
+}
+
+/*
  * Filling and flushing are done in batch, on arrays of void *s.  For filling,
  * the arrays go forward, and can be accessed with ordinary array arithmetic.
  * For flushing, we work from the end backwards, and so need to use special
@@ -463,6 +533,27 @@ cache_bin_finish_flush(cache_bin_t *bin, cache_bin_info_t *info,
 	cache_bin_low_water_adjust(bin);
 }
 
+static inline void
+cache_bin_init_ptr_array_for_stashed(cache_bin_t *bin, szind_t binind,
+    cache_bin_info_t *info, cache_bin_ptr_array_t *arr,
+    cache_bin_sz_t nstashed) {
+	assert(nstashed > 0);
+	assert(cache_bin_nstashed_get(bin, info) == nstashed);
+
+	void **full = cache_bin_full_position_get(bin, info);
+	arr->ptr = full;
+	assert(*arr->ptr != NULL);
+}
+
+static inline void
+cache_bin_finish_flush_stashed(cache_bin_t *bin, cache_bin_info_t *info) {
+	void **full = cache_bin_full_position_get(bin, info);
+
+	/* Reset the bin local full position. */
+	bin->low_bits_full = (uint16_t)(uintptr_t)full;
+	assert(cache_bin_nstashed_get(bin, info) == 0);
+}
+
 /*
  * Initialize a cache_bin_info to represent up to the given number of items in
  * the cache_bins it is associated with.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -415,6 +415,9 @@
 /* Performs additional size checks when defined. */
 #undef JEMALLOC_OPT_SIZE_CHECKS
 
+/* Allows sampled junk and stash for checking use-after-free when defined. */
+#undef JEMALLOC_UAF_DETECTION
+
 /* Darwin VM_MAKE_TAG support */
 #undef JEMALLOC_HAVE_VM_MAKE_TAG
 

include/jemalloc/internal/jemalloc_internal_externs.h

@@ -35,6 +35,9 @@ extern const char *zero_realloc_mode_names[];
 extern atomic_zu_t zero_realloc_count;
 extern bool opt_cache_oblivious;
 
+/* Escape free-fastpath when ptr & mask == 0 (for sanitization purpose). */
+extern uintptr_t san_cache_bin_nonfast_mask;
+
 /* Number of CPUs. */
 extern unsigned ncpus;
 

include/jemalloc/internal/jemalloc_preamble.h.in

@@ -198,6 +198,14 @@ static const bool config_opt_size_checks =
 #endif
     ;
 
+static const bool config_uaf_detection =
+#if defined(JEMALLOC_UAF_DETECTION) || defined(JEMALLOC_DEBUG)
+    true
+#else
+    false
+#endif
+    ;
+
 /* Whether or not the C++ extensions are enabled. */
 static const bool config_enable_cxx =
 #ifdef JEMALLOC_ENABLE_CXX

include/jemalloc/internal/san.h

@@ -10,9 +10,16 @@
 #define SAN_GUARD_LARGE_EVERY_N_EXTENTS_DEFAULT 0
 #define SAN_GUARD_SMALL_EVERY_N_EXTENTS_DEFAULT 0
 
+#define SAN_LG_UAF_ALIGN_DEFAULT (-1)
+#define SAN_CACHE_BIN_NONFAST_MASK_DEFAULT (uintptr_t)(-1)
+
+static const uintptr_t uaf_detect_junk = (uintptr_t)0x5b5b5b5b5b5b5b5bULL;
+
 /* 0 means disabled, i.e. never guarded. */
 extern size_t opt_san_guard_large;
 extern size_t opt_san_guard_small;
+/* -1 means disabled, i.e. never check for use-after-free. */
+extern ssize_t opt_lg_san_uaf_align;
 
 void san_guard_pages(tsdn_t *tsdn, ehooks_t *ehooks, edata_t *edata,
     emap_t *emap, bool left, bool right, bool remap);
@@ -24,7 +31,10 @@ void san_unguard_pages(tsdn_t *tsdn, ehooks_t *ehooks, edata_t *edata,
  */
 void san_unguard_pages_pre_destroy(tsdn_t *tsdn, ehooks_t *ehooks,
     edata_t *edata, emap_t *emap);
+void san_check_stashed_ptrs(void **ptrs, size_t nstashed, size_t usize);
+
 void tsd_san_init(tsd_t *tsd);
+void san_init(ssize_t lg_san_uaf_align);
 
 static inline void
 san_guard_pages_two_sided(tsdn_t *tsdn, ehooks_t *ehooks, edata_t *edata,
@@ -121,4 +131,62 @@ san_slab_extent_decide_guard(tsdn_t *tsdn, ehooks_t *ehooks) {
 	}
 }
 
+static inline void
+san_junk_ptr_locations(void *ptr, size_t usize, void **first, void **mid,
+    void **last) {
+	size_t ptr_sz = sizeof(void *);
+
+	*first = ptr;
+
+	*mid = (void *)((uintptr_t)ptr + ((usize >> 1) & ~(ptr_sz - 1)));
+	assert(*first != *mid || usize == ptr_sz);
+	assert((uintptr_t)*first <= (uintptr_t)*mid);
+
+	/*
+	 * When usize > 32K, the gap between requested_size and usize might be
+	 * greater than 4K -- this means the last write may access an
+	 * likely-untouched page (default settings w/ 4K pages).  However by
+	 * default the tcache only goes up to the 32K size class, and is usually
+	 * tuned lower instead of higher, which makes it less of a concern.
+	 */
+	*last = (void *)((uintptr_t)ptr + usize - sizeof(uaf_detect_junk));
+	assert(*first != *last || usize == ptr_sz);
+	assert(*mid != *last || usize <= ptr_sz * 2);
+	assert((uintptr_t)*mid <= (uintptr_t)*last);
+}
+
+static inline bool
+san_junk_ptr_should_slow(void) {
+	/*
+	 * The latter condition (pointer size greater than the min size class)
+	 * is not expected -- fall back to the slow path for simplicity.
+	 */
+	return config_debug || (LG_SIZEOF_PTR > SC_LG_TINY_MIN);
+}
+
+static inline void
+san_junk_ptr(void *ptr, size_t usize) {
+	if (san_junk_ptr_should_slow()) {
+		memset(ptr, (char)uaf_detect_junk, usize);
+		return;
+	}
+
+	void *first, *mid, *last;
+	san_junk_ptr_locations(ptr, usize, &first, &mid, &last);
+	*(uintptr_t *)first = uaf_detect_junk;
+	*(uintptr_t *)mid = uaf_detect_junk;
+	*(uintptr_t *)last = uaf_detect_junk;
+}
+
+static inline bool
+san_uaf_detection_enabled(void) {
+	bool ret = config_uaf_detection && (opt_lg_san_uaf_align != -1);
+	if (config_uaf_detection && ret) {
+		assert(san_cache_bin_nonfast_mask == ((uintptr_t)1 <<
+		    opt_lg_san_uaf_align) - 1);
+	}
+
+	return ret;
+}
+
 #endif /* JEMALLOC_INTERNAL_GUARD_H */

include/jemalloc/internal/tcache_externs.h

@@ -42,6 +42,8 @@ void	tcache_bin_flush_small(tsd_t *tsd, tcache_t *tcache, cache_bin_t *tbin,
     szind_t binind, unsigned rem);
 void tcache_bin_flush_large(tsd_t *tsd, tcache_t *tcache, cache_bin_t *tbin,
     szind_t binind, unsigned rem);
+void tcache_bin_flush_stashed(tsd_t *tsd, tcache_t *tcache, cache_bin_t *bin,
+    szind_t binind, bool is_small);
 void tcache_arena_reassociate(tsdn_t *tsdn, tcache_slow_t *tcache_slow,
     tcache_t *tcache, arena_t *arena);
 tcache_t *tcache_create_explicit(tsd_t *tsd);

include/jemalloc/internal/tcache_inlines.h

@@ -3,6 +3,7 @@
 
 #include "jemalloc/internal/bin.h"
 #include "jemalloc/internal/jemalloc_internal_types.h"
+#include "jemalloc/internal/san.h"
 #include "jemalloc/internal/sc.h"
 #include "jemalloc/internal/sz.h"
 #include "jemalloc/internal/util.h"
@@ -61,6 +62,8 @@ tcache_alloc_small(tsd_t *tsd, arena_t *arena, tcache_t *tcache,
 			return arena_malloc_hard(tsd_tsdn(tsd), arena, size,
 			    binind, zero);
 		}
+		tcache_bin_flush_stashed(tsd, tcache, bin, binind,
+		    /* is_small */ true);
 
 		ret = tcache_alloc_small_hard(tsd_tsdn(tsd), arena, tcache,
 		    bin, binind, &tcache_hard_success);
@@ -100,6 +103,8 @@ tcache_alloc_large(tsd_t *tsd, arena_t *arena, tcache_t *tcache, size_t size,
 		if (unlikely(arena == NULL)) {
 			return NULL;
 		}
+		tcache_bin_flush_stashed(tsd, tcache, bin, binind,
+		    /* is_small */ false);
 
 		ret = large_malloc(tsd_tsdn(tsd), arena, sz_s2u(size), zero);
 		if (ret == NULL) {
@@ -126,6 +131,21 @@ tcache_dalloc_small(tsd_t *tsd, tcache_t *tcache, void *ptr, szind_t binind,
 	assert(tcache_salloc(tsd_tsdn(tsd), ptr) <= SC_SMALL_MAXCLASS);
 
 	cache_bin_t *bin = &tcache->bins[binind];
+	/*
+	 * Not marking the branch unlikely because this is past free_fastpath()
+	 * (which handles the most common cases), i.e. at this point it's often
+	 * uncommon cases.
+	 */
+	if (cache_bin_nonfast_aligned(ptr)) {
+		/* Junk unconditionally, even if bin is full. */
+		san_junk_ptr(ptr, sz_index2size(binind));
+		if (cache_bin_stash(bin, ptr)) {
+			return;
+		}
+		assert(cache_bin_full(bin));
+		/* Bin full; fall through into the flush branch. */
+	}
+
 	if (unlikely(!cache_bin_dalloc_easy(bin, ptr))) {
 		if (unlikely(tcache_small_bin_disabled(binind, bin))) {
 			arena_dalloc_small(tsd_tsdn(tsd), ptr);

src/arena.c

@@ -157,6 +157,8 @@ arena_stats_merge(tsdn_t *tsdn, arena_t *arena, unsigned *nthreads,
 			cache_bin_t *cache_bin = &descriptor->bins[i];
 			astats->tcache_bytes +=
 			    cache_bin_ncached_get_remote(cache_bin,
+			    &tcache_bin_info[i]) * sz_index2size(i) +
+			    cache_bin_nstashed_get(cache_bin,
 			    &tcache_bin_info[i]) * sz_index2size(i);
 		}
 	}

src/cache_bin.c

@@ -2,6 +2,8 @@
 #include "jemalloc/internal/jemalloc_internal_includes.h"
 
 #include "jemalloc/internal/bit_util.h"
+#include "jemalloc/internal/cache_bin.h"
+#include "jemalloc/internal/safety_check.h"
 
 void
 cache_bin_info_init(cache_bin_info_t *info,

src/ctl.c

@@ -150,6 +150,7 @@ CTL_PROTO(opt_prof_recent_alloc_max)
 CTL_PROTO(opt_prof_stats)
 CTL_PROTO(opt_prof_sys_thread_name)
 CTL_PROTO(opt_prof_time_res)
+CTL_PROTO(opt_lg_san_uaf_align)
 CTL_PROTO(opt_zero_realloc)
 CTL_PROTO(tcache_create)
 CTL_PROTO(tcache_flush)
@@ -472,6 +473,7 @@ static const ctl_named_node_t opt_node[] = {
 	{NAME("prof_stats"),	CTL(opt_prof_stats)},
 	{NAME("prof_sys_thread_name"),	CTL(opt_prof_sys_thread_name)},
 	{NAME("prof_time_resolution"),	CTL(opt_prof_time_res)},
+	{NAME("lg_san_uaf_align"),	CTL(opt_lg_san_uaf_align)},
 	{NAME("zero_realloc"),	CTL(opt_zero_realloc)}
 };
 
@@ -2201,6 +2203,8 @@ CTL_RO_NL_CGEN(config_prof, opt_prof_sys_thread_name, opt_prof_sys_thread_name,
     bool)
 CTL_RO_NL_CGEN(config_prof, opt_prof_time_res,
     prof_time_res_mode_names[opt_prof_time_res], const char *)
+CTL_RO_NL_CGEN(config_uaf_detection, opt_lg_san_uaf_align,
+    opt_lg_san_uaf_align, ssize_t)
 CTL_RO_NL_GEN(opt_zero_realloc,
     zero_realloc_mode_names[opt_zero_realloc_action], const char *)
 

src/jemalloc.c

@@ -1657,6 +1657,31 @@ malloc_conf_init_helper(sc_data_t *sc_data, unsigned bin_shard_sizes[SC_NBINS],
 				}
 				CONF_CONTINUE;
 			}
+			if (config_uaf_detection &&
+			    CONF_MATCH("lg_san_uaf_align")) {
+				ssize_t a;
+				CONF_VALUE_READ(ssize_t, a)
+				if (CONF_VALUE_READ_FAIL() || a < -1) {
+					CONF_ERROR("Invalid conf value",
+					    k, klen, v, vlen);
+				}
+				if (a == -1) {
+					opt_lg_san_uaf_align = -1;
+					CONF_CONTINUE;
+				}
+
+				/* clip if necessary */
+				ssize_t max_allowed = (sizeof(size_t) << 3) - 1;
+				ssize_t min_allowed = LG_PAGE;
+				if (a > max_allowed) {
+					a = max_allowed;
+				} else if (a < min_allowed) {
+					a = min_allowed;
+				}
+
+				opt_lg_san_uaf_align = a;
+				CONF_CONTINUE;
+			}
 
 			CONF_HANDLE_SIZE_T(opt_san_guard_small,
 			    "san_guard_small", 0, SIZE_T_MAX,
@@ -1760,6 +1785,7 @@ malloc_init_hard_a0_locked() {
 		prof_boot0();
 	}
 	malloc_conf_init(&sc_data, bin_shard_sizes);
+	san_init(opt_lg_san_uaf_align);
 	sz_boot(&sc_data, opt_cache_oblivious);
 	bin_info_boot(&sc_data, bin_shard_sizes);
 
@@ -2970,6 +2996,41 @@ free_default(void *ptr) {
 	}
 }
 
+JEMALLOC_ALWAYS_INLINE bool
+free_fastpath_nonfast_aligned(void *ptr, bool check_prof) {
+	/*
+	 * free_fastpath do not handle two uncommon cases: 1) sampled profiled
+	 * objects and 2) sampled junk & stash for use-after-free detection.
+	 * Both have special alignments which are used to escape the fastpath.
+	 *
+	 * prof_sample is page-aligned, which covers the UAF check when both
+	 * are enabled (the assertion below).  Avoiding redundant checks since
+	 * this is on the fastpath -- at most one runtime branch from this.
+	 */
+	if (config_debug && cache_bin_nonfast_aligned(ptr)) {
+		assert(prof_sample_aligned(ptr));
+	}
+
+	if (config_prof && check_prof) {
+		/* When prof is enabled, the prof_sample alignment is enough. */
+		if (prof_sample_aligned(ptr)) {
+			return true;
+		} else {
+			return false;
+		}
+	}
+
+	if (config_uaf_detection) {
+		if (cache_bin_nonfast_aligned(ptr)) {
+			return true;
+		} else {
+			return false;
+		}
+	}
+
+	return false;
+}
+
 /* Returns whether or not the free attempt was successful. */
 JEMALLOC_ALWAYS_INLINE
 bool free_fastpath(void *ptr, size_t size, bool size_hint) {
@@ -2992,18 +3053,21 @@ bool free_fastpath(void *ptr, size_t size, bool size_hint) {
 		    &arena_emap_global, ptr, &alloc_ctx);
 
 		/* Note: profiled objects will have alloc_ctx.slab set */
-		if (unlikely(err || !alloc_ctx.slab)) {
+		if (unlikely(err || !alloc_ctx.slab ||
+		    free_fastpath_nonfast_aligned(ptr,
+		    /* check_prof */ false))) {
 			return false;
 		}
 		assert(alloc_ctx.szind != SC_NSIZES);
 	} else {
 		/*
-		 * Check for both sizes that are too large, and for sampled
+		 * Check for both sizes that are too large, and for sampled /
-		 * objects.  Sampled objects are always page-aligned.  The
+		 * special aligned objects.  The alignment check will also check
-		 * sampled object check will also check for null ptr.
+		 * for null ptr.
 		 */
 		if (unlikely(size > SC_LOOKUP_MAXCLASS ||
-		    (config_prof && prof_sample_aligned(ptr)))) {
+		    free_fastpath_nonfast_aligned(ptr,
+		    /* check_prof */ true))) {
 			return false;
 		}
 		alloc_ctx.szind = sz_size2index_lookup(size);

src/san.c

@@ -10,6 +10,15 @@
 size_t opt_san_guard_large = SAN_GUARD_LARGE_EVERY_N_EXTENTS_DEFAULT;
 size_t opt_san_guard_small = SAN_GUARD_SMALL_EVERY_N_EXTENTS_DEFAULT;
 
+/* Aligned (-1 is off) ptrs will be junked & stashed on dealloc. */
+ssize_t opt_lg_san_uaf_align = SAN_LG_UAF_ALIGN_DEFAULT;
+
+/*
+ *  Initialized in san_init().  When disabled, the mask is set to (uintptr_t)-1
+ *  to always fail the nonfast_align check.
+ */
+uintptr_t san_cache_bin_nonfast_mask = SAN_CACHE_BIN_NONFAST_MASK_DEFAULT;
+
 static inline void
 san_find_guarded_addr(edata_t *edata, uintptr_t *guard1, uintptr_t *guard2,
     uintptr_t *addr, size_t size, bool left, bool right) {
@@ -141,8 +150,59 @@ san_unguard_pages_pre_destroy(tsdn_t *tsdn, ehooks_t *ehooks, edata_t *edata,
 	    /* right */ true, /* remap */ false);
 }
 
+static bool
+san_stashed_corrupted(void *ptr, size_t size) {
+	if (san_junk_ptr_should_slow()) {
+		for (size_t i = 0; i < size; i++) {
+			if (((char *)ptr)[i] != (char)uaf_detect_junk) {
+				return true;
+			}
+		}
+		return false;
+	}
+
+	void *first, *mid, *last;
+	san_junk_ptr_locations(ptr, size, &first, &mid, &last);
+	if (*(uintptr_t *)first != uaf_detect_junk ||
+	    *(uintptr_t *)mid != uaf_detect_junk ||
+	    *(uintptr_t *)last != uaf_detect_junk) {
+		return true;
+	}
+
+	return false;
+}
+
+void
+san_check_stashed_ptrs(void **ptrs, size_t nstashed, size_t usize) {
+	/*
+	 * Verify that the junked-filled & stashed pointers remain unchanged, to
+	 * detect write-after-free.
+	 */
+	for (size_t n = 0; n < nstashed; n++) {
+		void *stashed = ptrs[n];
+		assert(stashed != NULL);
+		assert(cache_bin_nonfast_aligned(stashed));
+		if (unlikely(san_stashed_corrupted(stashed, usize))) {
+			safety_check_fail("<jemalloc>: Write-after-free "
+			    "detected on deallocated pointer %p (size %zu).\n",
+			    stashed, usize);
+		}
+	}
+}
+
 void
 tsd_san_init(tsd_t *tsd) {
 	*tsd_san_extents_until_guard_smallp_get(tsd) = opt_san_guard_small;
 	*tsd_san_extents_until_guard_largep_get(tsd) = opt_san_guard_large;
 }
+
+void
+san_init(ssize_t lg_san_uaf_align) {
+	assert(lg_san_uaf_align == -1 || lg_san_uaf_align >= LG_PAGE);
+	if (lg_san_uaf_align == -1) {
+		san_cache_bin_nonfast_mask = (uintptr_t)-1;
+		return;
+	}
+
+	san_cache_bin_nonfast_mask = ((uintptr_t)1 << lg_san_uaf_align) - 1;
+}

src/tcache.c

@@ -4,6 +4,7 @@
 #include "jemalloc/internal/assert.h"
 #include "jemalloc/internal/mutex.h"
 #include "jemalloc/internal/safety_check.h"
+#include "jemalloc/internal/san.h"
 #include "jemalloc/internal/sc.h"
 
 /******************************************************************************/
@@ -179,6 +180,8 @@ tcache_event(tsd_t *tsd) {
 	bool is_small = (szind < SC_NBINS);
 	cache_bin_t *cache_bin = &tcache->bins[szind];
 
+	tcache_bin_flush_stashed(tsd, tcache, cache_bin, szind, is_small);
+
 	cache_bin_sz_t low_water = cache_bin_low_water_get(cache_bin,
 	    &tcache_bin_info[szind]);
 	if (low_water > 0) {
@@ -497,6 +500,8 @@ tcache_bin_flush_impl(tsd_t *tsd, tcache_t *tcache, cache_bin_t *cache_bin,
 JEMALLOC_ALWAYS_INLINE void
 tcache_bin_flush_bottom(tsd_t *tsd, tcache_t *tcache, cache_bin_t *cache_bin,
     szind_t binind, unsigned rem, bool small) {
+	tcache_bin_flush_stashed(tsd, tcache, cache_bin, binind, small);
+
 	cache_bin_sz_t ncached = cache_bin_ncached_get_local(cache_bin,
 	    &tcache_bin_info[binind]);
 	assert((cache_bin_sz_t)rem <= ncached);
@@ -525,6 +530,48 @@ tcache_bin_flush_large(tsd_t *tsd, tcache_t *tcache, cache_bin_t *cache_bin,
 	tcache_bin_flush_bottom(tsd, tcache, cache_bin, binind, rem, false);
 }
 
+/*
+ * Flushing stashed happens when 1) tcache fill, 2) tcache flush, or 3) tcache
+ * GC event.  This makes sure that the stashed items do not hold memory for too
+ * long, and new buffers can only be allocated when nothing is stashed.
+ *
+ * The downside is, the time between stash and flush may be relatively short,
+ * especially when the request rate is high.  It lowers the chance of detecting
+ * write-after-free -- however that is a delayed detection anyway, and is less
+ * of a focus than the memory overhead.
+ */
+void
+tcache_bin_flush_stashed(tsd_t *tsd, tcache_t *tcache, cache_bin_t *cache_bin,
+    szind_t binind, bool is_small) {
+	cache_bin_info_t *info = &tcache_bin_info[binind];
+	/*
+	 * The two below are for assertion only.  The content of original cached
+	 * items remain unchanged -- the stashed items reside on the other end
+	 * of the stack.  Checking the stack head and ncached to verify.
+	 */
+	void *head_content = *cache_bin->stack_head;
+	cache_bin_sz_t orig_cached = cache_bin_ncached_get_local(cache_bin,
+	    info);
+
+	cache_bin_sz_t nstashed = cache_bin_nstashed_get(cache_bin, info);
+	assert(orig_cached + nstashed <= cache_bin_info_ncached_max(info));
+	if (nstashed == 0) {
+		return;
+	}
+
+	CACHE_BIN_PTR_ARRAY_DECLARE(ptrs, nstashed);
+	cache_bin_init_ptr_array_for_stashed(cache_bin, binind, info, &ptrs,
+	    nstashed);
+	san_check_stashed_ptrs(ptrs.ptr, nstashed, sz_index2size(binind));
+	tcache_bin_flush_impl(tsd, tcache, cache_bin, binind, &ptrs, nstashed,
+	    is_small);
+	cache_bin_finish_flush_stashed(cache_bin, info);
+
+	assert(cache_bin_nstashed_get(cache_bin, info) == 0);
+	assert(cache_bin_ncached_get_local(cache_bin, info) == orig_cached);
+	assert(head_content == *cache_bin->stack_head);
+}
+
 void
 tcache_arena_associate(tsdn_t *tsdn, tcache_slow_t *tcache_slow,
     tcache_t *tcache, arena_t *arena) {

test/include/test/arena_util.h

@@ -26,6 +26,12 @@ do_arena_create(ssize_t dirty_decay_ms, ssize_t muzzy_decay_ms) {
 
 static inline void
 do_arena_destroy(unsigned arena_ind) {
+	/* 
+	 * For convenience, flush tcache in case there are cached items.
+	 * However not assert success since the tcache may be disabled.
+	 */
+	mallctl("thread.tcache.flush", NULL, NULL, NULL, 0);
+
 	size_t mib[3];
 	size_t miblen = sizeof(mib)/sizeof(size_t);
 	expect_d_eq(mallctlnametomib("arena.0.destroy", mib, &miblen), 0,

test/unit/cache_bin.c

@@ -82,27 +82,30 @@ do_batch_alloc_test(cache_bin_t *bin, cache_bin_info_t *info, void **ptrs,
 	free(out);
 }
 
+static void
+test_bin_init(cache_bin_t *bin, cache_bin_info_t *info) {
+	size_t size;
+	size_t alignment;
+	cache_bin_info_compute_alloc(info, 1, &size, &alignment);
+	void *mem = mallocx(size, MALLOCX_ALIGN(alignment));
+	assert_ptr_not_null(mem, "Unexpected mallocx failure");
+
+	size_t cur_offset = 0;
+	cache_bin_preincrement(info, 1, mem, &cur_offset);
+	cache_bin_init(bin, info, mem, &cur_offset);
+	cache_bin_postincrement(info, 1, mem, &cur_offset);
+	assert_zu_eq(cur_offset, size, "Should use all requested memory");
+}
+
 TEST_BEGIN(test_cache_bin) {
 	const int ncached_max = 100;
 	bool success;
 	void *ptr;
 
-	cache_bin_t bin;
 	cache_bin_info_t info;
 	cache_bin_info_init(&info, ncached_max);
-
+	cache_bin_t bin;
-	size_t size;
+	test_bin_init(&bin, &info);
-	size_t alignment;
-	cache_bin_info_compute_alloc(&info, 1, &size, &alignment);
-	void *mem = mallocx(size, MALLOCX_ALIGN(alignment));
-	assert_ptr_not_null(mem, "Unexpected mallocx failure");
-
-	size_t cur_offset = 0;
-	cache_bin_preincrement(&info, 1, mem, &cur_offset);
-	cache_bin_init(&bin, &info, mem, &cur_offset);
-	cache_bin_postincrement(&info, 1, mem, &cur_offset);
-
-	assert_zu_eq(cur_offset, size, "Should use all requested memory");
 
 	/* Initialize to empty; should then have 0 elements. */
 	expect_d_eq(ncached_max, cache_bin_info_ncached_max(&info), "");
@@ -258,7 +261,123 @@ TEST_BEGIN(test_cache_bin) {
 }
 TEST_END
 
+static void
+do_flush_stashed_test(cache_bin_t *bin, cache_bin_info_t *info, void **ptrs,
+    cache_bin_sz_t nfill, cache_bin_sz_t nstash) {
+	expect_true(cache_bin_ncached_get_local(bin, info) == 0,
+	    "Bin not empty");
+	expect_true(cache_bin_nstashed_get(bin, info) == 0, "Bin not empty");
+	expect_true(nfill + nstash <= info->ncached_max, "Exceeded max");
+
+	bool ret;
+	/* Fill */
+	for (cache_bin_sz_t i = 0; i < nfill; i++) {
+		ret = cache_bin_dalloc_easy(bin, &ptrs[i]);
+		expect_true(ret, "Unexpected fill failure");
+	}
+	expect_true(cache_bin_ncached_get_local(bin, info) == nfill,
+	    "Wrong cached count");
+
+	/* Stash */
+	for (cache_bin_sz_t i = 0; i < nstash; i++) {
+		ret = cache_bin_stash(bin, &ptrs[i + nfill]);
+		expect_true(ret, "Unexpected stash failure");
+	}
+	expect_true(cache_bin_nstashed_get(bin, info) == nstash,
+	    "Wrong stashed count");
+
+	if (nfill + nstash == info->ncached_max) {
+		ret = cache_bin_dalloc_easy(bin, &ptrs[0]);
+		expect_false(ret, "Should not dalloc into a full bin");
+		ret = cache_bin_stash(bin, &ptrs[0]);
+		expect_false(ret, "Should not stash into a full bin");
+	}
+
+	/* Alloc filled ones */
+	for (cache_bin_sz_t i = 0; i < nfill; i++) {
+		void *ptr = cache_bin_alloc(bin, &ret);
+		expect_true(ret, "Unexpected alloc failure");
+		/* Verify it's not from the stashed range. */
+		expect_true((uintptr_t)ptr < (uintptr_t)&ptrs[nfill],
+		    "Should not alloc stashed ptrs");
+	}
+	expect_true(cache_bin_ncached_get_local(bin, info) == 0,
+	    "Wrong cached count");
+	expect_true(cache_bin_nstashed_get(bin, info) == nstash,
+	    "Wrong stashed count");
+
+	cache_bin_alloc(bin, &ret);
+	expect_false(ret, "Should not alloc stashed");
+
+	/* Clear stashed ones */
+	cache_bin_finish_flush_stashed(bin, info);
+	expect_true(cache_bin_ncached_get_local(bin, info) == 0,
+	    "Wrong cached count");
+	expect_true(cache_bin_nstashed_get(bin, info) == 0,
+	    "Wrong stashed count");
+
+	cache_bin_alloc(bin, &ret);
+	expect_false(ret, "Should not alloc from empty bin");
+}
+
+TEST_BEGIN(test_cache_bin_stash) {
+	const int ncached_max = 100;
+
+	cache_bin_t bin;
+	cache_bin_info_t info;
+	cache_bin_info_init(&info, ncached_max);
+	test_bin_init(&bin, &info);
+
+	/*
+	 * The content of this array is not accessed; instead the interior
+	 * addresses are used to insert / stash into the bins as test pointers.
+	 */
+	void **ptrs = mallocx(sizeof(void *) * (ncached_max + 1), 0);
+	assert_ptr_not_null(ptrs, "Unexpected mallocx failure");
+	bool ret;
+	for (cache_bin_sz_t i = 0; i < ncached_max; i++) {
+		expect_true(cache_bin_ncached_get_local(&bin, &info) ==
+		    (i / 2 + i % 2), "Wrong ncached value");
+		expect_true(cache_bin_nstashed_get(&bin, &info) == i / 2,
+		    "Wrong nstashed value");
+		if (i % 2 == 0) {
+			cache_bin_dalloc_easy(&bin, &ptrs[i]);
+		} else {
+			ret = cache_bin_stash(&bin, &ptrs[i]);
+			expect_true(ret, "Should be able to stash into a "
+			    "non-full cache bin");
+		}
+	}
+	ret = cache_bin_dalloc_easy(&bin, &ptrs[0]);
+	expect_false(ret, "Should not dalloc into a full cache bin");
+	ret = cache_bin_stash(&bin, &ptrs[0]);
+	expect_false(ret, "Should not stash into a full cache bin");
+	for (cache_bin_sz_t i = 0; i < ncached_max; i++) {
+		void *ptr = cache_bin_alloc(&bin, &ret);
+		if (i < ncached_max / 2) {
+			expect_true(ret, "Should be able to alloc");
+			uintptr_t diff = ((uintptr_t)ptr - (uintptr_t)&ptrs[0])
+			    / sizeof(void *);
+			expect_true(diff % 2 == 0, "Should be able to alloc");
+		} else {
+			expect_false(ret, "Should not alloc stashed");
+			expect_true(cache_bin_nstashed_get(&bin, &info) ==
+			    ncached_max / 2, "Wrong nstashed value");
+		}
+	}
+
+	test_bin_init(&bin, &info);
+	do_flush_stashed_test(&bin, &info, ptrs, ncached_max, 0);
+	do_flush_stashed_test(&bin, &info, ptrs, 0, ncached_max);
+	do_flush_stashed_test(&bin, &info, ptrs, ncached_max / 2, ncached_max / 2);
+	do_flush_stashed_test(&bin, &info, ptrs, ncached_max / 4, ncached_max / 2);
+	do_flush_stashed_test(&bin, &info, ptrs, ncached_max / 2, ncached_max / 4);
+	do_flush_stashed_test(&bin, &info, ptrs, ncached_max / 4, ncached_max / 4);
+}
+TEST_END
+
 int
 main(void) {
-	return test(test_cache_bin);
+	return test(test_cache_bin,
+		test_cache_bin_stash);
 }

test/unit/mallctl.c

@@ -323,6 +323,7 @@ TEST_BEGIN(test_mallctl_opt) {
 	TEST_MALLCTL_OPT(ssize_t, prof_recent_alloc_max, prof);
 	TEST_MALLCTL_OPT(bool, prof_stats, prof);
 	TEST_MALLCTL_OPT(bool, prof_sys_thread_name, prof);
+	TEST_MALLCTL_OPT(ssize_t, lg_san_uaf_align, uaf_detection);
 
 #undef TEST_MALLCTL_OPT
 }
@@ -368,7 +369,7 @@ TEST_BEGIN(test_tcache_none) {
 	/* Make sure that tcache-based allocation returns p, not q. */
 	void *p1 = mallocx(42, 0);
 	expect_ptr_not_null(p1, "Unexpected mallocx() failure");
-	if (!opt_prof) {
+	if (!opt_prof && !san_uaf_detection_enabled()) {
 		expect_ptr_eq(p0, p1,
 		    "Expected tcache to allocate cached region");
 	}
@@ -434,8 +435,10 @@ TEST_BEGIN(test_tcache) {
 		ps[i] = mallocx(psz, MALLOCX_TCACHE(tis[i]));
 		expect_ptr_not_null(ps[i], "Unexpected mallocx() failure, i=%u",
 		    i);
-		expect_ptr_eq(ps[i], p0,
+		if (!san_uaf_detection_enabled()) {
-		    "Expected mallocx() to allocate cached region, i=%u", i);
+			expect_ptr_eq(ps[i], p0, "Expected mallocx() to "
+			    "allocate cached region, i=%u", i);
+		}
 	}
 
 	/* Verify that reallocation uses cached regions. */
@@ -444,8 +447,10 @@ TEST_BEGIN(test_tcache) {
 		qs[i] = rallocx(ps[i], qsz, MALLOCX_TCACHE(tis[i]));
 		expect_ptr_not_null(qs[i], "Unexpected rallocx() failure, i=%u",
 		    i);
-		expect_ptr_eq(qs[i], q0,
+		if (!san_uaf_detection_enabled()) {
-		    "Expected rallocx() to allocate cached region, i=%u", i);
+			expect_ptr_eq(qs[i], q0, "Expected rallocx() to "
+			    "allocate cached region, i=%u", i);
+		}
 		/* Avoid undefined behavior in case of test failure. */
 		if (qs[i] == NULL) {
 			qs[i] = ps[i];

test/unit/tcache_max.c

@@ -152,6 +152,7 @@ TEST_BEGIN(test_tcache_max) {
 	test_skip_if(!config_stats);
 	test_skip_if(!opt_tcache);
 	test_skip_if(opt_prof);
+	test_skip_if(san_uaf_detection_enabled());
 
 	for (alloc_option = alloc_option_start;
 	     alloc_option < alloc_option_end;

test/unit/tcache_max.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-export MALLOC_CONF="tcache_max:1024"
+export MALLOC_CONF="tcache_max:1024,lg_san_uaf_align:-1"

test/unit/uaf.c

@@ -0,0 +1,225 @@
+#include "test/jemalloc_test.h"
+#include "test/arena_util.h"
+
+#include "jemalloc/internal/cache_bin.h"
+#include "jemalloc/internal/safety_check.h"
+
+static size_t san_uaf_align;
+
+static bool fake_abort_called;
+void fake_abort(const char *message) {
+	(void)message;
+	fake_abort_called = true;
+}
+
+static void
+test_write_after_free_pre(void) {
+	safety_check_set_abort(&fake_abort);
+	fake_abort_called = false;
+}
+
+static void
+test_write_after_free_post(void) {
+	assert_d_eq(mallctl("thread.tcache.flush", NULL, NULL, NULL, 0),
+	    0, "Unexpected tcache flush failure");
+	expect_true(fake_abort_called, "Use-after-free check didn't fire.");
+	safety_check_set_abort(NULL);
+}
+
+static bool
+uaf_detection_enabled(void) {
+	if (!config_uaf_detection) {
+		return false;
+	}
+
+	ssize_t lg_san_uaf_align;
+	size_t sz = sizeof(lg_san_uaf_align);
+	assert_d_eq(mallctl("opt.lg_san_uaf_align", &lg_san_uaf_align, &sz,
+	    NULL, 0), 0, "Unexpected mallctl failure");
+	if (lg_san_uaf_align < 0) {
+		return false;
+	}
+	assert_zd_ge(lg_san_uaf_align, LG_PAGE, "san_uaf_align out of range");
+	san_uaf_align = (size_t)1 << lg_san_uaf_align;
+
+	bool tcache_enabled;
+	sz = sizeof(tcache_enabled);
+	assert_d_eq(mallctl("thread.tcache.enabled", &tcache_enabled, &sz, NULL,
+	    0), 0, "Unexpected mallctl failure");
+	if (!tcache_enabled) {
+		return false;
+	}
+
+	return true;
+}
+
+static void
+test_use_after_free(size_t alloc_size, bool write_after_free) {
+	void *ptr = (void *)(uintptr_t)san_uaf_align;
+	assert_true(cache_bin_nonfast_aligned(ptr), "Wrong alignment");
+	ptr = (void *)((uintptr_t)123 * (uintptr_t)san_uaf_align);
+	assert_true(cache_bin_nonfast_aligned(ptr), "Wrong alignment");
+	ptr = (void *)((uintptr_t)san_uaf_align + 1);
+	assert_false(cache_bin_nonfast_aligned(ptr), "Wrong alignment");
+
+	/*
+	 * Disable purging (-1) so that all dirty pages remain committed, to
+	 * make use-after-free tolerable.
+	 */
+	unsigned arena_ind = do_arena_create(-1, -1);
+	int flags = MALLOCX_ARENA(arena_ind) | MALLOCX_TCACHE_NONE;
+
+	size_t n_max = san_uaf_align * 2;
+	void **items = mallocx(n_max * sizeof(void *), flags);
+	assert_ptr_not_null(items, "Unexpected mallocx failure");
+
+	bool found = false;
+	size_t iter = 0;
+	char magic = 's';
+	assert_d_eq(mallctl("thread.tcache.flush", NULL, NULL, NULL, 0),
+	    0, "Unexpected tcache flush failure");
+	while (!found) {
+		ptr = mallocx(alloc_size, flags);
+		assert_ptr_not_null(ptr, "Unexpected mallocx failure");
+
+		found = cache_bin_nonfast_aligned(ptr);
+		*(char *)ptr = magic;
+		items[iter] = ptr;
+		assert_zu_lt(iter++, n_max, "No aligned ptr found");
+	}
+
+	if (write_after_free) {
+		test_write_after_free_pre();
+	}
+	bool junked = false;
+	while (iter-- != 0) {
+		char *volatile mem = items[iter];
+		assert_c_eq(*mem, magic, "Unexpected memory content");
+		free(mem);
+		if (*mem != magic) {
+			junked = true;
+			assert_c_eq(*mem, (char)uaf_detect_junk,
+			    "Unexpected junk-filling bytes");
+			if (write_after_free) {
+				*(char *)mem = magic + 1;
+			}
+		}
+		/* Flush tcache (including stashed). */
+		assert_d_eq(mallctl("thread.tcache.flush", NULL, NULL, NULL, 0),
+		    0, "Unexpected tcache flush failure");
+	}
+	expect_true(junked, "Aligned ptr not junked");
+	if (write_after_free) {
+		test_write_after_free_post();
+	}
+
+	dallocx(items, flags);
+	do_arena_destroy(arena_ind);
+}
+
+TEST_BEGIN(test_read_after_free) {
+	test_skip_if(!uaf_detection_enabled());
+
+	test_use_after_free(sizeof(void *), /* write_after_free */ false);
+	test_use_after_free(sizeof(void *) + 1, /* write_after_free */ false);
+	test_use_after_free(16, /* write_after_free */ false);
+	test_use_after_free(20, /* write_after_free */ false);
+	test_use_after_free(32, /* write_after_free */ false);
+	test_use_after_free(33, /* write_after_free */ false);
+	test_use_after_free(48, /* write_after_free */ false);
+	test_use_after_free(64, /* write_after_free */ false);
+	test_use_after_free(65, /* write_after_free */ false);
+	test_use_after_free(129, /* write_after_free */ false);
+	test_use_after_free(255, /* write_after_free */ false);
+	test_use_after_free(256, /* write_after_free */ false);
+}
+TEST_END
+
+TEST_BEGIN(test_write_after_free) {
+	test_skip_if(!uaf_detection_enabled());
+
+	test_use_after_free(sizeof(void *), /* write_after_free */ true);
+	test_use_after_free(sizeof(void *) + 1, /* write_after_free */ true);
+	test_use_after_free(16, /* write_after_free */ true);
+	test_use_after_free(20, /* write_after_free */ true);
+	test_use_after_free(32, /* write_after_free */ true);
+	test_use_after_free(33, /* write_after_free */ true);
+	test_use_after_free(48, /* write_after_free */ true);
+	test_use_after_free(64, /* write_after_free */ true);
+	test_use_after_free(65, /* write_after_free */ true);
+	test_use_after_free(129, /* write_after_free */ true);
+	test_use_after_free(255, /* write_after_free */ true);
+	test_use_after_free(256, /* write_after_free */ true);
+}
+TEST_END
+
+static bool
+check_allocated_intact(void **allocated, size_t n_alloc) {
+	for (unsigned i = 0; i < n_alloc; i++) {
+		void *ptr = *(void **)allocated[i];
+		bool found = false;
+		for (unsigned j = 0; j < n_alloc; j++) {
+			if (ptr == allocated[j]) {
+				found = true;
+				break;
+			}
+		}
+		if (!found) {
+			return false;
+		}
+	}
+
+	return true;
+}
+
+TEST_BEGIN(test_use_after_free_integration) {
+	test_skip_if(!uaf_detection_enabled());
+
+	unsigned arena_ind = do_arena_create(-1, -1);
+	int flags = MALLOCX_ARENA(arena_ind);
+
+	size_t n_alloc = san_uaf_align * 2;
+	void **allocated = mallocx(n_alloc * sizeof(void *), flags);
+	assert_ptr_not_null(allocated, "Unexpected mallocx failure");
+
+	for (unsigned i = 0; i < n_alloc; i++) {
+		allocated[i] = mallocx(sizeof(void *) * 8, flags);
+		assert_ptr_not_null(allocated[i], "Unexpected mallocx failure");
+		if (i > 0) {
+			/* Emulate a circular list. */
+			*(void **)allocated[i] = allocated[i - 1];
+		}
+	}
+	*(void **)allocated[0] = allocated[n_alloc - 1];
+	expect_true(check_allocated_intact(allocated, n_alloc),
+	    "Allocated data corrupted");
+
+	for (unsigned i = 0; i < n_alloc; i++) {
+		free(allocated[i]);
+	}
+	/* Read-after-free */
+	expect_false(check_allocated_intact(allocated, n_alloc),
+	    "Junk-filling not detected");
+
+	test_write_after_free_pre();
+	for (unsigned i = 0; i < n_alloc; i++) {
+		allocated[i] = mallocx(sizeof(void *), flags);
+		assert_ptr_not_null(allocated[i], "Unexpected mallocx failure");
+		*(void **)allocated[i] = (void *)(uintptr_t)i;
+	}
+	/* Write-after-free */
+	for (unsigned i = 0; i < n_alloc; i++) {
+		free(allocated[i]);
+		*(void **)allocated[i] = NULL;
+	}
+	test_write_after_free_post();
+}
+TEST_END
+
+int
+main(void) {
+	return test(
+	    test_read_after_free,
+	    test_write_after_free,
+	    test_use_after_free_integration);
+}

test/unit/uaf.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+export MALLOC_CONF="lg_san_uaf_align:12"