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Fix race in free() of fastbin chunk: BZ #15073

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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.
This commit is contained in:
Maxim Kuvyrkov
2013-12-24 09:44:50 +13:00
parent b9bcbbcbe7
commit 362b47fe09
3 changed files with 30 additions and 19 deletions
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20
malloc/malloc.c
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@ -3783,25 +3783,29 @@ _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;