commit 362b47fe09ca9a928d444c7e2f7992f7f61bfc3e
Author: Maxim Kuvyrkov <maxim@kugelworks.com>
Date:   Tue Dec 24 09:44:50 2013 +1300

    Fix race in free() of fastbin chunk: BZ #15073
    
    Perform sanity check only if we have_lock.  Due to lockless nature of fastbins
    we need to be careful derefencing pointers to fastbin entries (chunksize(old)
    in this case) in multithreaded environments.
    
    The fix is to add have_lock to the if-condition checks.  The rest of the patch
    only makes code more readable.
    
    	* malloc/malloc.c (_int_free): Perform sanity check only if we
    	have_lock.

Index: b/malloc/malloc.c
===================================================================
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -3909,25 +3909,29 @@ _int_free(mstate av, mchunkptr p, int ha
     unsigned int idx = fastbin_index(size);
     fb = &fastbin (av, idx);
 
-    mchunkptr fd;
-    mchunkptr old = *fb;
+    /* Atomically link P to its fastbin: P->FD = *FB; *FB = P;  */
+    mchunkptr old = *fb, old2;
     unsigned int old_idx = ~0u;
     do
       {
-	/* Another simple check: make sure the top of the bin is not the
-	   record we are going to add (i.e., double free).  */
+	/* Check that the top of the bin is not the record we are going to add
+	   (i.e., double free).  */
 	if (__builtin_expect (old == p, 0))
 	  {
 	    errstr = "double free or corruption (fasttop)";
 	    goto errout;
 	  }
-	if (old != NULL)
+	/* Check that size of fastbin chunk at the top is the same as
+	   size of the chunk that we are adding.  We can dereference OLD
+	   only if we have the lock, otherwise it might have already been
+	   deallocated.  See use of OLD_IDX below for the actual check.  */
+	if (have_lock && old != NULL)
 	  old_idx = fastbin_index(chunksize(old));
-	p->fd = fd = old;
+	p->fd = old2 = old;
       }
-    while ((old = catomic_compare_and_exchange_val_rel (fb, p, fd)) != fd);
+    while ((old = catomic_compare_and_exchange_val_rel (fb, p, old2)) != old2);
 
-    if (fd != NULL && __builtin_expect (old_idx != idx, 0))
+    if (have_lock && old != NULL && __builtin_expect (old_idx != idx, 0))
       {
 	errstr = "invalid fastbin entry (free)";
 	goto errout;