[PATCH] xfs: reject log continuation for a header-only item in recovery

[Xiang Mei <xmei5@asu.edu>]

On an XLOG_WAS_CONT_TRANS op, xlog_recover_add_to_cont_trans() extends the
tail item's last region via item->ri_buf[item->ri_cnt-1] without checking
that the item has one. A header-only item, created by
xlog_recover_add_to_trans() when a transaction's first op is a bare
sizeof(struct xfs_trans_header) op carrying XFS_TRANS_HEADER_MAGIC, sits on
r_itemq with ri_cnt == 0 and ri_buf == NULL. A crafted log whose ops are
XLOG_START_TRANS, that header op, then XLOG_WAS_CONT_TRANS thus
dereferences ((struct kvec *)NULL)[-1] and faults during mount(2) log
recovery of an untrusted XFS image.

A valid XLOG_WAS_CONT_TRANS only follows a region written with
XLOG_CONTINUE_TRANS, so ri_cnt >= 1 always holds for a real log. Reject the
ri_cnt == 0 case as corruption with -EFSCORRUPTED, which the caller already
propagates to abort recovery.

  BUG: unable to handle page fault for address: fffffffffffffff0
  #PF: supervisor read access in kernel mode
  Oops: 0000 [#1] SMP KASAN NOPTI
  RIP: 0010:xlog_recover_add_to_cont_trans (fs/xfs/xfs_log_recover.c:2135)
  Call Trace:
   xlog_recovery_process_trans (fs/xfs/xfs_log_recover.c:2306)
   xlog_recover_process_data (fs/xfs/xfs_log_recover.c:2500)
   xlog_do_recovery_pass (fs/xfs/xfs_log_recover.c:3243)
   xlog_do_log_recovery (fs/xfs/xfs_log_recover.c:3331)
   xlog_do_recover (fs/xfs/xfs_log_recover.c:3368)
   xlog_recover (fs/xfs/xfs_log_recover.c:3493)
   xfs_log_mount (fs/xfs/xfs_log.c:617)
   xfs_mountfs (fs/xfs/xfs_mount.c:1031)
   xfs_fs_fill_super (fs/xfs/xfs_super.c:1940)
   get_tree_bdev_flags (fs/super.c:1634)
   vfs_get_tree (fs/super.c:1694)
   fc_mount (fs/namespace.c:1198)
   path_mount (fs/namespace.c:4161)
   __x64_sys_mount (fs/namespace.c:4367)
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  Kernel panic - not syncing: Fatal exception

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@asu.edu>
---
 fs/xfs/xfs_log_recover.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 5f984bf5698a..fa8e04bf1aac 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -2131,6 +2131,9 @@ xlog_recover_add_to_cont_trans(
 	item = list_entry(trans->r_itemq.prev, struct xlog_recover_item,
 			  ri_list);
 
+	if (item->ri_cnt == 0)
+		return -EFSCORRUPTED;
+
 	old_ptr = item->ri_buf[item->ri_cnt-1].iov_base;
 	old_len = item->ri_buf[item->ri_cnt-1].iov_len;
 
-- 
2.43.0