[PATCH v2] ext4: return an error when a journal block is not mapped

[Karl Mehltretter <kmehltretter@gmail.com>]

If the internal journal inode contains a hole, ext4_map_blocks() returns
0. ext4_journal_bmap() logs "journal bmap failed" and aborts the
journal, but also returns 0 and leaves *block unchanged.

jbd2_journal_bmap() takes that as success and uses the unchanged logical
journal block number as a physical filesystem block number. Journal I/O
can then overwrite filesystem blocks outside the journal despite the
abort.

Return -EFSCORRUPTED for a hole, matching the error passed to
jbd2_journal_abort(). Keep propagating negative mapping errors.

Fixes: 62913ae96de7 ("ext4, jbd2: add an optimized bmap for the journal inode")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
---
Changes in v2:
- Assign -EFSCORRUPTED to ret before aborting and returning (Markus, Jan).
- Add Reviewed-by tags from Jan Kara and Zhang Yi.
- Rebase onto v7.3-rc7.

v1: https://lore.kernel.org/r/20261004180629.27213-1-kmehltretter@gmail.com/

Found with an experimental clang that warns when a function returns 0
right after reporting a failure.

Recorded QEMU A/B testing used x86_64 kernels and a 1 KiB-block image
with a hole in the journal inode starting at logical block 40. The
unpatched kernel overwrote filesystem block 40, a reserved GDT block.
With the patch, none of the marker blocks changed. There were three
runs per kernel.

Both kernels logged the mapping failure and journal abort:

  EXT4-fs (vda): journal bmap failed: block 40 ret 0
  Aborting journal on device vda-8.

 fs/ext4/super.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index d636b89090a2..1e959f196ffe 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5967,7 +5967,9 @@ static int ext4_journal_bmap(journal_t *journal, sector_t *block)
 		ext4_msg(journal->j_inode->i_sb, KERN_CRIT,
 			 "journal bmap failed: block %llu ret %d\n",
 			 *block, ret);
-		jbd2_journal_abort(journal, ret ? ret : -EFSCORRUPTED);
+		if (!ret)
+			ret = -EFSCORRUPTED;
+		jbd2_journal_abort(journal, ret);
 		return ret;
 	}
 	*block = map.m_pblk;

base-commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907
-- 
2.39.5 (Apple Git-154)