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 3de21fd1270e8d7ee97506f9acd8ba09a1fac7ce..d823b1100c1b4551b00a7bd9f9b4ee02e4ce3c6c 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.
+
 2012-11-20  Thomas Schwinge  <thomas@codesourcery.com>
 
        * sysdeps/sh/dl-machine.h (ELF_MACHINE_RUNTIME_FIXUP_PARAMS): New

diff --git a/NEWS b/NEWS
index 86c6dfcbba8f287f79449ccf6a83dc827403191d..2c5fb07f7bccf08d28d8b9b53f0933aa249c8979 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -9,7 +9,7 @@ Version 2.16.1
 
 * The following bugs are resolved with this release:
 
-  6530, 14195, 14459, 14476, 14562, 14621, 14648, 14756, 14831
+  6530, 14195, 14459, 14476, 14562, 14621, 14648, 14756, 14831, 15073
 
 Version 2.16
 

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 28039b4720dfa973a9577ca35a8f78e2ade65fb4..0ee5676c2e55a6db192d7180af312c851b31d65f 100644 (file)
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -3868,25 +3868,29 @@ _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;