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.

Conflicts:

	ChangeLog
	NEWS

3 files changed, 20 insertions, 9 deletions

diff --git a/ChangeLog b/ChangeLog
index 1e5efa74c0..74d433076e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2014-01-04  Maxim Kuvyrkov  <maxim@kugelworks.com>
+	    Ondřej Bílka  <neleai@seznam.cz>
+
+	[BZ #15073]
+	* malloc/malloc.c (_int_free): Perform sanity check only if we
+        have_lock.
+
 2013-11-11  David S. Miller  <davem@davemloft.net>
 
 	[BZ #16150]
diff --git a/NEWS b/NEWS
index 6c7aaa7c21..29856cc046 100644
--- a/NEWS
+++ b/NEWS
@@ -9,7 +9,7 @@ Version 2.18.1
 
 * The following bugs are resolved with this release:
 
-  15128, 15909, 15996, 16150.
+  15073, 15128, 15909, 15996, 16150.
 
 Version 2.18
 
diff --git a/malloc/malloc.c b/malloc/malloc.c
index be472b2ba3..8a0eb854e2 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -3809,25 +3809,29 @@ _int_free(mstate av, mchunkptr p, int have_lock)
     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;