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

diff --git a/ChangeLog b/ChangeLog
index b76e7dc06e5bf8e890be069ada67a5b995dac61f..49bf8f90010613b5dadc0a5ac5d4ba7d16d4250f 100644 (file)
--- 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.
+
 2013-02-08  Carlos O'Donell  <carlos@redhat.com>
 
        [BZ #15006]

diff --git a/NEWS b/NEWS
index 10c1ca62acd65f3af548a924db386fee0f23964c..c9cf487624db564c102de853156363f358a18cca 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -8,7 +8,7 @@ using `glibc' in the "product" field.
 Version 2.17.1
 
 * The following bugs are resolved with this release:
-  15003, 15006, 15122, 15759.
+  15003, 15006, 15073, 15122, 15759.
 
 \f
 Version 2.17

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 3f0b6b1cd4b341ddc46c8a15213595f62bd66a76..acf3d4202f1dc2752e3bc5893779d7069dcbc36f 100644 (file)
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -3816,25 +3816,29 @@ _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;