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.

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;
+}