ext4: avoid potential buffer over-read in parse_apply_sb_mount_options()

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: 8b67f04 ("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>

Author: tytso

fs/ext4/super.c

@@ -2469,23 +2469,19 @@ static int parse_apply_sb_mount_options(struct super_block *sb,
 					struct ext4_fs_context *m_ctx)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
-	char *s_mount_opts = NULL;
+	char s_mount_opts[65];
 	struct ext4_fs_context *s_ctx = NULL;
 	struct fs_context *fc = NULL;
 	int ret = -ENOMEM;
 
 	if (!sbi->s_es->s_mount_opts[0])
 		return 0;
 
-	s_mount_opts = kstrndup(sbi->s_es->s_mount_opts,
-				sizeof(sbi->s_es->s_mount_opts),
-				GFP_KERNEL);
-	if (!s_mount_opts)
-		return ret;
+	strscpy_pad(s_mount_opts, sbi->s_es->s_mount_opts);
 
 	fc = kzalloc(sizeof(struct fs_context), GFP_KERNEL);
 	if (!fc)
-		goto out_free;
+		return -ENOMEM;
 
 	s_ctx = kzalloc(sizeof(struct ext4_fs_context), GFP_KERNEL);
 	if (!s_ctx)
@@ -2517,11 +2513,8 @@ static int parse_apply_sb_mount_options(struct super_block *sb,
 	ret = 0;
 
 out_free:
-	if (fc) {
-		ext4_fc_free(fc);
-		kfree(fc);
-	}
-	kfree(s_mount_opts);
+	ext4_fc_free(fc);
+	kfree(fc);
 	return ret;
 }