Re: [PATCH 1/3] ext4: avoid potential buffer over-read in parse_apply_sb_mount_options()

On Mon, Sep 08, 2025 at 11:15:48PM -0400, Theodore Ts'o via B4 Relay wrote:

From: Theodore Ts'o tytso@mit.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.

Uh.... does that mean that a filesystem with exactly 64 bytes worth of mount option string (and no trailing null) could do something malicious?

My guess is that s_usr_quota_inum mostly saves us, but a nastycrafted filesystem with more than 2^24 inodes could cause an out of bounds memory access? But that most likely will just fail the mount option parser anyway?

--D

Cc: stable@vger.kernel.org Fixes: 8b67f04ab9de ("ext4: Add mount options in superblock") Signed-off-by: Theodore Ts'o tytso@mit.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