Re: [RFC PATCH] vfs: exclude ntfs3 from file mode validation in may_open()

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Aug 11, 2025 at 04:05:51PM +0900, Tetsuo Handa wrote:
> Since ntfs_read_mft() not only accepts file modes which may_open() accepts
> but also accepts
> 
>   (fname && fname->home.low == cpu_to_le32(MFT_REC_EXTEND) &&
>    fname->home.seq == cpu_to_le16(MFT_REC_EXTEND)
> 
> case when the file mode is none of
> S_IFDIR/S_IFLNK/S_IFREG/S_IFCHR/S_IFBLK/S_IFIFO/S_IFSOCK, may_open() cannot
> unconditionally expect IS_ANON_FILE(inode) when the file mode is none of
> S_IFDIR/S_IFLNK/S_IFREG/S_IFCHR/S_IFBLK/S_IFIFO/S_IFSOCK.
> 
> Treat as if S_IFREG when the inode is for NTFS3 filesystem.
> 
> Reported-by: syzbot <syzbot+895c23f6917da440ed0d@syzkaller.appspotmail.com>
> Closes: https://syzkaller.appspot.com/bug?extid=895c23f6917da440ed0d
> Fixes: af153bb63a33 ("vfs: catch invalid modes in may_open()")
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
> Is it possible to handle this problem on the NTFS3 side?
> 
>   --- a/fs/ntfs3/inode.c
>   +++ b/fs/ntfs3/inode.c
>   @@ -470,8 +470,9 @@ static struct inode *ntfs_read_mft(struct inode *inode,
>           } else if (fname && fname->home.low == cpu_to_le32(MFT_REC_EXTEND) &&
>                      fname->home.seq == cpu_to_le16(MFT_REC_EXTEND)) {
>                   /* Records in $Extend are not a files or general directories. */
>                   inode->i_op = &ntfs_file_inode_operations;
>   +               mode = S_IFREG;
>           } else {
>                   err = -EINVAL;
>                   goto out;
>           }
> 
> I don't know what breaks if we pretend as if S_IFREG...
> 
>  fs/namei.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index cd43ff89fbaa..a66599754394 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3471,6 +3471,12 @@ static int may_open(struct mnt_idmap *idmap, const struct path *path,
>  			return -EACCES;
>  		break;
>  	default:
> +		/* Special handling for ntfs_read_mft() case. */
> +		if (inode->i_sb->s_magic == 0x7366746e) {
> +			if ((acc_mode & MAY_EXEC) && path_noexec(path))
> +				return -EACCES;
> +			break;
> +		}
>  		VFS_BUG_ON_INODE(!IS_ANON_FILE(inode), inode);
>  	}
>  

Bravo, but that's several months late - would be lovely for an AFD posting.
In the current form the patch is obviously unacceptable; if we do that
kind of special-casing, the proper place is in register_filesystem() where
we clearly ought to compare fs->name with "ntfs" and return an error.

All jokes aside, this kind of stuff is a non-starter.  Blacklist it on
syzbot side or fix fs/ntfs3; VFS is *NOT* the place for this kind of
special-casing.