xfs: fix log block underflow during recovery cycle verification

[ Upstream commit 9f2a4505800607e537e9dd9dea4f55c4b0c30c7a ]

It is possible for mkfs to format very small filesystems with too
small of an internal log with respect to the various minimum size
and block count requirements. If this occurs when the log happens to
be smaller than the scan window used for cycle verification and the
scan wraps the end of the log, the start_blk calculation in
xlog_find_head() underflows and leads to an attempt to scan an
invalid range of log blocks. This results in log recovery failure
and a failed mount.

Since there may be filesystems out in the wild with this kind of
geometry, we cannot simply refuse to mount. Instead, cap the scan
window for cycle verification to the size of the physical log. This
ensures that the cycle verification proceeds as expected when the
scan wraps the end of the log.

Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>

diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 4aefff89949d64fac9b79ca17b2ba29096200da5..c0065697498c7be88e4ad796a4fbfdf8bd05427c 100644 (file)
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -738,7 +738,7 @@ xlog_find_head(
         * in the in-core log.  The following number can be made tighter if
         * we actually look at the block size of the filesystem.
         */
-       num_scan_bblks = XLOG_TOTAL_REC_SHIFT(log);
+       num_scan_bblks = min_t(int, log_bbnum, XLOG_TOTAL_REC_SHIFT(log));
        if (head_blk >= num_scan_bblks) {
                /*
                 * We are guaranteed that the entire check can be performed