[PATCH] xfs: ensure log recovery buffer is resized to avoid OOB

[Raphael Pinsonneault-Thibeault]

In xlog_do_recovery_pass(),
commit 45cf976008dd ("xfs: fix log recovery buffer allocation for the legacy h_size fixup")
added a fix to take the corrected h_size (from the xfsprogs bug
workaround) into consideration for the log recovery buffer calculation.
Without it, we would still allocate the buffer based on the incorrect
on-disk size.

However, in a scenario similar to 45cf976008dd, syzbot creates a fuzzed
record where xfs_has_logv2() but the xlog_rec_header h_version !=
XLOG_VERSION_2. Meaning, we skip the log recover buffer calculation
fix and allocate the buffer based on the incorrect on-disk size. Hence,
a KASAN: slab-out-of-bounds read in xlog_do_recovery_pass() ->
xlog_recover_process() -> xlog_cksum().

Fix by removing the check for xlog_rec_header h_version, since the code
is already within the if(xfs_has_logv2) path. The CRC checksum will
reject the bad record anyway, this fix is to ensure we can read the
entire buffer without an OOB.

Reported-by: syzbot+9f6d080dece587cfdd4c@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=9f6d080dece587cfdd4c
Tested-by: syzbot+9f6d080dece587cfdd4c@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 45cf976008dd ("xfs: fix log recovery buffer allocation for the legacy h_size fixup")
Signed-off-by: Raphael Pinsonneault-Thibeault <rpthibeault@xxxxxxxxx>
---

Can xfs_has_logv2() and xlog_rec_header h_version ever disagree?

>From my understanding, whenever the h_version is set (e.g. in
xlog_add_record()), it is set to 2 when xfs_has_logv2(), and 1
otherwise.

 fs/xfs/xfs_log_recover.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index e6ed9e09c027..08d7b25c4ab1 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -3064,8 +3064,7 @@ xlog_do_recovery_pass(
 		 * still allocate the buffer based on the incorrect on-disk
 		 * size.
 		 */
-		if (h_size > XLOG_HEADER_CYCLE_SIZE &&
-		    (rhead->h_version & cpu_to_be32(XLOG_VERSION_2))) {
+		if (h_size > XLOG_HEADER_CYCLE_SIZE) {
 			hblks = DIV_ROUND_UP(h_size, XLOG_HEADER_CYCLE_SIZE);
 			if (hblks > 1) {
 				kvfree(hbp);
-- 
2.43.0