FAILED: patch "[PATCH] jbd2: fix integer underflow in" failed to apply to 5.10-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x 289a2ca0c9b7eae74f93fc213b0b971669b8683d
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026072041-pork-untapped-b293@gregkh' --subject-prefix 'PATCH 5.10.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 289a2ca0c9b7eae74f93fc213b0b971669b8683d Mon Sep 17 00:00:00 2001
From: Junrui Luo <moonafterrain@outlook.com>
Date: Wed, 13 May 2026 17:28:40 +0800
Subject: [PATCH] jbd2: fix integer underflow in
 jbd2_journal_initialize_fast_commit()

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@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
Fixes: e029c5f27987 ("ext4: make num of fast commit blocks configurable")
Reviewed-by: Baokun Li <libaokun@linux.alibaba.com>
Fixes: e029c5f279872 ("ext4: make num of fast commit blocks configurable")
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/SYBPR01MB7881663C927DE9D7BBF4D1DFAF062@SYBPR01MB7881.ausprd01.prod.outlook.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 4f397fcdb13c..e3b2e38e1a1b 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;