[PATCH v2 3/3] xfs: fail recovery on a committed log item with no regions

[Weiming Shi <bestswngs@gmail.com>]

If the first op of a transaction is a bare transaction header
(len == sizeof(struct xfs_trans_header)), xlog_recover_add_to_trans()
adds a recovery item but no region, leaving it on r_itemq with
ri_cnt == 0 and ri_buf == NULL.

If a commit op follows, xlog_recover_reorder_trans() calls ITEM_TYPE()
on the item, which reads *(unsigned short *)item->ri_buf[0].iov_base and
faults on the NULL ri_buf.

The log is CRC-checked, so this op sequence comes from a crafted image,
not media corruption.  It faults the kernel when such an image is
mounted:

 KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
 RIP: 0010:xlog_recover_reorder_trans (fs/xfs/xfs_log_recover.c:1836)
  xlog_recover_commit_trans (fs/xfs/xfs_log_recover.c:2043)
  xlog_recover_process_data (fs/xfs/xfs_log_recover.c:2501)
  xlog_do_recovery_pass (fs/xfs/xfs_log_recover.c:3244)
  xlog_recover (fs/xfs/xfs_log_recover.c:3493)
  xfs_log_mount (fs/xfs/xfs_log.c:618)
  xfs_mountfs (fs/xfs/xfs_mount.c:1034)
  xfs_fs_fill_super (fs/xfs/xfs_super.c:1938)
  vfs_get_tree (fs/super.c:1695)
  path_mount (fs/namespace.c:4161)
  __x64_sys_mount (fs/namespace.c:4367)

A committed item always carries its format descriptor in ri_buf[0], so
one with no regions is invalid.  Reject it with -EFSCORRUPTED, like the
unrecognized item type below.

Fixes: 89cebc847729 ("xfs: validate transaction header length on log recovery")
Reported-by: Xiang Mei <xmei5@asu.edu>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 fs/xfs/xfs_log_recover.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 5347f6a5ec42..461e847c32a2 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -1907,6 +1907,15 @@ xlog_recover_reorder_trans(
 	list_for_each_entry_safe(item, n, &sort_list, ri_list) {
 		enum xlog_recover_reorder	fate = XLOG_REORDER_ITEM_LIST;
 
+		/* a committed item with no regions has a NULL ri_buf[0] */
+		if (!item->ri_cnt || !item->ri_buf) {
+			xfs_warn(log->l_mp,
+				"%s: committed log item has no regions",
+				__func__);
+			error = -EFSCORRUPTED;
+			break;
+		}
+
 		item->ri_ops = xlog_find_item_ops(item);
 		if (!item->ri_ops) {
 			xfs_warn(log->l_mp,
-- 
2.43.0