On Mon, Sep 08, 2025 at 11:15:48PM -0400, Theodore Ts'o via B4 Relay wrote:
> From: Theodore Ts'o <tytso@xxxxxxx>
>
> 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@xxxxxxxxxxxxxxx
> Fixes: 8b67f04ab9de ("ext4: Add mount options in superblock")
> Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
> ---
> 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
>
>
>
