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commit 362b47fe09ca9a928d444c7e2f7992f7f61bfc3e
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Author: Maxim Kuvyrkov <maxim@kugelworks.com>
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Date: Tue Dec 24 09:44:50 2013 +1300
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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
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we need to be careful derefencing pointers to fastbin entries (chunksize(old)
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in this case) in multithreaded environments.
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The fix is to add have_lock to the if-condition checks. The rest of the patch
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only makes code more readable.
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* malloc/malloc.c (_int_free): Perform sanity check only if we
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have_lock.
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Index: b/malloc/malloc.c
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===================================================================
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--- a/malloc/malloc.c
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+++ b/malloc/malloc.c
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@@ -3909,25 +3909,29 @@ _int_free(mstate av, mchunkptr p, int ha
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unsigned int idx = fastbin_index(size);
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fb = &fastbin (av, idx);
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- mchunkptr fd;
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- mchunkptr old = *fb;
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+ /* Atomically link P to its fastbin: P->FD = *FB; *FB = P; */
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+ mchunkptr old = *fb, old2;
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unsigned int old_idx = ~0u;
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do
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{
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- /* Another simple check: make sure the top of the bin is not the
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- record we are going to add (i.e., double free). */
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+ /* Check that the top of the bin is not the record we are going to add
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+ (i.e., double free). */
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if (__builtin_expect (old == p, 0))
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{
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errstr = "double free or corruption (fasttop)";
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goto errout;
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}
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- if (old != NULL)
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+ /* Check that size of fastbin chunk at the top is the same as
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+ size of the chunk that we are adding. We can dereference OLD
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+ only if we have the lock, otherwise it might have already been
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+ deallocated. See use of OLD_IDX below for the actual check. */
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+ if (have_lock && old != NULL)
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old_idx = fastbin_index(chunksize(old));
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- p->fd = fd = old;
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+ p->fd = old2 = old;
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}
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- while ((old = catomic_compare_and_exchange_val_rel (fb, p, fd)) != fd);
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+ while ((old = catomic_compare_and_exchange_val_rel (fb, p, old2)) != old2);
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- if (fd != NULL && __builtin_expect (old_idx != idx, 0))
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+ if (have_lock && old != NULL && __builtin_expect (old_idx != idx, 0))
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{
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errstr = "invalid fastbin entry (free)";
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goto errout;
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