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[PATCH] malloc: Validate tc_idx before checking for double-frees in tcache [BZ #23907]


The previous check could read beyond the end of the tcache entry
array.  If the e->key == tcache cookie check happened to pass, this
would result in crashes.

2018-11-22  Florian Weimer  <fweimer@redhat.com>

	[BZ #23907]
	* malloc/malloc.c (_int_free): Validate tc_idx before checking for
	double-frees.

diff --git a/malloc/malloc.c b/malloc/malloc.c
index f730d7a2ee..c9b2c6e320 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -4225,33 +4225,33 @@ _int_free (mstate av, mchunkptr p, int have_lock)
 #if USE_TCACHE
   {
     size_t tc_idx = csize2tidx (size);
-
-    /* Check to see if it's already in the tcache.  */
-    tcache_entry *e = (tcache_entry *) chunk2mem (p);
-
-    /* This test succeeds on double free.  However, we don't 100%
-       trust it (it also matches random payload data at a 1 in
-       2^<size_t> chance), so verify it's not an unlikely coincidence
-       before aborting.  */
-    if (__glibc_unlikely (e->key == tcache && tcache))
+    if (tcache != NULL && tc_idx < mp_.tcache_bins)
       {
-	tcache_entry *tmp;
-	LIBC_PROBE (memory_tcache_double_free, 2, e, tc_idx);
-	for (tmp = tcache->entries[tc_idx];
-	     tmp;
-	     tmp = tmp->next)
-	  if (tmp == e)
-	    malloc_printerr ("free(): double free detected in tcache 2");
-	/* If we get here, it was a coincidence.  We've wasted a few
-	   cycles, but don't abort.  */
-      }
+	/* Check to see if it's already in the tcache.  */
+	tcache_entry *e = (tcache_entry *) chunk2mem (p);
 
-    if (tcache
-	&& tc_idx < mp_.tcache_bins
-	&& tcache->counts[tc_idx] < mp_.tcache_count)
-      {
-	tcache_put (p, tc_idx);
-	return;
+	/* This test succeeds on double free.  However, we don't 100%
+	   trust it (it also matches random payload data at a 1 in
+	   2^<size_t> chance), so verify it's not an unlikely
+	   coincidence before aborting.  */
+	if (__glibc_unlikely (e->key == tcache))
+	  {
+	    tcache_entry *tmp;
+	    LIBC_PROBE (memory_tcache_double_free, 2, e, tc_idx);
+	    for (tmp = tcache->entries[tc_idx];
+		 tmp;
+		 tmp = tmp->next)
+	      if (tmp == e)
+		malloc_printerr ("free(): double free detected in tcache 2");
+	    /* If we get here, it was a coincidence.  We've wasted a
+	       few cycles, but don't abort.  */
+	  }
+
+	if (tcache->counts[tc_idx] < mp_.tcache_count)
+	  {
+	    tcache_put (p, tc_idx);
+	    return;
+	  }
       }
   }
 #endif


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