[PATCH 5.4.y] ext4: avoid potential buffer over-read in parse_apply_sb_mount_options()

[Sasha Levin <sashal@kernel.org>]

From: Theodore Ts'o <tytso@mit.edu>

[ Upstream commit 8ecb790ea8c3fc69e77bace57f14cf0d7c177bd8 ]

Unlike other strings in the ext4 superblock, we rely on tune2fs to
make sure s_mount_opts is NUL terminated.  Harden
parse_apply_sb_mount_options() by treating s_mount_opts as a potential
__nonstring.

Cc: stable@vger.kernel.org
Fixes: 8b67f04ab9de ("ext4: Add mount options in superblock")
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Message-ID: <20250916-tune2fs-v2-1-d594dc7486f0@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[ applied to ext4_fill_super() instead of parse_apply_sb_mount_options() ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/ext4/super.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index ff681888a123f..0c7aedcb39ea4 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3882,18 +3882,16 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	}
 
 	if (sbi->s_es->s_mount_opts[0]) {
-		char *s_mount_opts = kstrndup(sbi->s_es->s_mount_opts,
-					      sizeof(sbi->s_es->s_mount_opts),
-					      GFP_KERNEL);
-		if (!s_mount_opts)
-			goto failed_mount;
+		char s_mount_opts[65];
+
+		strscpy_pad(s_mount_opts, sbi->s_es->s_mount_opts,
+			    sizeof(s_mount_opts));
 		if (!parse_options(s_mount_opts, sb, &journal_devnum,
 				   &journal_ioprio, 0)) {
 			ext4_msg(sb, KERN_WARNING,
 				 "failed to parse options in superblock: %s",
 				 s_mount_opts);
 		}
-		kfree(s_mount_opts);
 	}
 	sbi->s_def_mount_opt = sbi->s_mount_opt;
 	if (!parse_options((char *) data, sb, &journal_devnum,
-- 
2.51.0