[PATCH v2] jbd2: fix integer underflow in jbd2_journal_initialize_fast_commit()

[Junrui Luo]

jbd2_journal_initialize_fast_commit() validates journal capacity by
checking (journal->j_last - num_fc_blks < JBD2_MIN_JOURNAL_BLOCKS).
Both j_last and num_fc_blks are unsigned, so when num_fc_blks exceeds
j_last the subtraction wraps to a large value, bypassing the bounds
check.

The resulting underflow corrupts j_last, j_fc_first, and j_free,
leading to journal abort.

Fix by checking num_fc_blks against j_last before the subtraction,
returning -EFSCORRUPTED.

Fixes: 6866d7b3f2bb ("ext4 / jbd2: add fast commit initialization")
Reported-by: Yuhao Jiang <danisjiang@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Junrui Luo <moonafterrain@xxxxxxxxxxx>
---
Changes in v2:
- Return -EFSCORRUPTED instead of -ENOSPC
- Link to v1: https://lore.kernel.org/all/SYBPR01MB78813DD23B28BD49B1AA1123AF392@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
---
 fs/jbd2/journal.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index cb2c529a8f1b..0bb97459fbf0 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -2263,6 +2263,8 @@ jbd2_journal_initialize_fast_commit(journal_t *journal)
 	unsigned long long num_fc_blks;
 
 	num_fc_blks = jbd2_journal_get_num_fc_blks(sb);
+	if (num_fc_blks > journal->j_last)
+		return -EFSCORRUPTED;
 	if (journal->j_last - num_fc_blks < JBD2_MIN_JOURNAL_BLOCKS)
 		return -ENOSPC;
 

---
base-commit: 7aaa8047eafd0bd628065b15757d9b48c5f9c07d
change-id: 20260513-fixes-e6dcda3273d4

Best regards,
-- 
Junrui Luo <moonafterrain@xxxxxxxxxxx>