ext4: fix use-after-free in ext4_mb_new_blocks
ext4_mb_put_pa should hold pa->pa_lock before accessing pa->pa_count.
While ext4_mb_use_preallocated checks pa->pa_deleted first and then
increments pa->count later, ext4_mb_put_pa decrements pa->pa_count
before holding pa->pa_lock and then sets pa->pa_deleted.
* Free sequence
ext4_mb_put_pa (1): atomic_dec_and_test pa->pa_count
ext4_mb_put_pa (2): lock pa->pa_lock
ext4_mb_put_pa (3): check pa->pa_deleted
ext4_mb_put_pa (4): set pa->pa_deleted=1
ext4_mb_put_pa (5): unlock pa->pa_lock
ext4_mb_put_pa (6): remove pa from a list
ext4_mb_pa_callback: free pa
* Use sequence
ext4_mb_use_preallocated (1): iterate over preallocation
ext4_mb_use_preallocated (2): lock pa->pa_lock
ext4_mb_use_preallocated (3): check pa->pa_deleted
ext4_mb_use_preallocated (4): increase pa->pa_count
ext4_mb_use_preallocated (5): unlock pa->pa_lock
ext4_mb_release_context: access pa
* Use-after-free sequence
[initial status] <pa->pa_deleted = 0, pa_count = 1>
ext4_mb_use_preallocated (1): iterate over preallocation
ext4_mb_use_preallocated (2): lock pa->pa_lock
ext4_mb_use_preallocated (3): check pa->pa_deleted
ext4_mb_put_pa (1): atomic_dec_and_test pa->pa_count
[pa_count decremented] <pa->pa_deleted = 0, pa_count = 0>
ext4_mb_use_preallocated (4): increase pa->pa_count
[pa_count incremented] <pa->pa_deleted = 0, pa_count = 1>
ext4_mb_use_preallocated (5): unlock pa->pa_lock
ext4_mb_put_pa (2): lock pa->pa_lock
ext4_mb_put_pa (3): check pa->pa_deleted
ext4_mb_put_pa (4): set pa->pa_deleted=1
[race condition!] <pa->pa_deleted = 1, pa_count = 1>
ext4_mb_put_pa (5): unlock pa->pa_lock
ext4_mb_put_pa (6): remove pa from a list
ext4_mb_pa_callback: free pa
ext4_mb_release_context: access pa
AddressSanitizer has detected use-after-free in ext4_mb_new_blocks
Bug report: http://goo.gl/rG1On3
Signed-off-by: Junho Ryu <jayr@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 4d113ef..04766d9 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -3442,6 +3442,9 @@
{
struct ext4_prealloc_space *pa;
pa = container_of(head, struct ext4_prealloc_space, u.pa_rcu);
+
+ BUG_ON(atomic_read(&pa->pa_count));
+ BUG_ON(pa->pa_deleted == 0);
kmem_cache_free(ext4_pspace_cachep, pa);
}
@@ -3455,11 +3458,13 @@
ext4_group_t grp;
ext4_fsblk_t grp_blk;
- if (!atomic_dec_and_test(&pa->pa_count) || pa->pa_free != 0)
- return;
-
/* in this short window concurrent discard can set pa_deleted */
spin_lock(&pa->pa_lock);
+ if (!atomic_dec_and_test(&pa->pa_count) || pa->pa_free != 0) {
+ spin_unlock(&pa->pa_lock);
+ return;
+ }
+
if (pa->pa_deleted == 1) {
spin_unlock(&pa->pa_lock);
return;