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

["Darrick J. Wong" <djwong@kernel.org>]

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
> 
> 
>