linux-ext4 - [PATCH v2 1/3] ext4: avoid potential buffer over-read in parse_apply_sb_mount

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Message-Id: <20250916-tune2fs-v2-1-d594dc7486f0@mit.edu>
Date: Tue, 16 Sep 2025 23:22:47 -0400
From: Theodore Ts'o via B4 Relay <devnull+tytso.mit.edu@...nel.org>
To: tytso@....edu
Cc: linux-ext4@...r.kernel.org, linux-api@...r.kernel.org, 
 stable@...r.kernel.org
Subject: [PATCH v2 1/3] ext4: avoid potential buffer over-read in
 parse_apply_sb_mount_options()

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

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@...r.kernel.org
Fixes: 8b67f04ab9de ("ext4: Add mount options in superblock")
Signed-off-by: Theodore Ts'o <tytso@....edu>
---
 fs/ext4/super.c | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 699c15db28a82f26809bf68533454a242596f0fd..94c98446c84f9a4614971d246ca7f001de610a8a 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2460,7 +2460,7 @@ 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;
@@ -2468,15 +2468,11 @@ static int parse_apply_sb_mount_options(struct super_block *sb,
 	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)
@@ -2508,11 +2504,8 @@ 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;
 }
 

-- 
2.51.0