From 3db0119ef56decc402827d5b84774fa4487f6cd4 Mon Sep 17 00:00:00 2001 From: Maxim Kuvyrkov Date: Tue, 24 Dec 2013 09:44:50 +1300 Subject: [PATCH] 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 --- ChangeLog | 7 +++++++ NEWS | 2 +- malloc/malloc.c | 20 ++++++++++++-------- 3 files changed, 20 insertions(+), 9 deletions(-) diff --git a/ChangeLog b/ChangeLog index b76e7dc06e5..49bf8f90010 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,10 @@ +2014-01-04 Maxim Kuvyrkov + Ondřej Bílka + + [BZ #15073] + * malloc/malloc.c (_int_free): Perform sanity check only if we + have_lock. + 2013-02-08 Carlos O'Donell [BZ #15006] diff --git a/NEWS b/NEWS index 10c1ca62acd..c9cf487624d 100644 --- 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. Version 2.17 diff --git a/malloc/malloc.c b/malloc/malloc.c index 3f0b6b1cd4b..acf3d4202f1 100644 --- 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; -- 2.47.2