Date:   Tue, 13 Mar 2018 11:39:31 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org,
        containers@...ts.linux-foundation.org
Subject: Re: [PATCH 2/4 v5] devpts: resolve devpts bind-mounts

Christian Brauner <christian.brauner@...ntu.com> writes:

Reviewed-by: "Eric W. Biederman" <ebiederm@...ssion.com>

It would have been more readable if Linus's suggested cleanup were
in a separate patch.  But unless you need to respin I would not worry
about that.

> Most libcs will still look at /dev/ptmx when opening the master fd of a pty
> device. When /dev/ptmx is a bind-mount of /dev/pts/ptmx and the TIOCGPTPEER
> ioctl() is used to safely retrieve a file descriptor for the slave side of
> the pty based on the master fd, the /proc/self/fd/{0,1,2} symlinks will
> point to /. A very simply reproducer for this issue presupposing a libc
> that uses TIOCGPTPEER in its openpty() implementation is:
>
> unshare --mount
> mount --bind /dev/pts/ptmx /dev/ptmx
> chmod 666 /dev/ptmx
> script
> ls -al /proc/self/fd/0
>
> Having bind-mounts of /dev/pts/ptmx to /dev/ptmx not working correctly is a
> regression. In addition, it is also a fairly common scenario in containers
> employing user namespaces.
>
> The reason for the current failure is that the kernel tries to verify the
> useability of the devpts filesystem without resolving the /dev/ptmx
> bind-mount first. This will lead it to detect that the dentry is escaping
> its bind-mount. The reason is that while the devpts filesystem mounted at
> /dev/pts has the devtmpfs mounted at /dev as its parent mount:
>
> 21 -- -- / /dev
> -- 21 -- / /dev/pts
>
> devtmpfs and devpts are on different devices
>
> -- -- 0:6  / /dev
> -- -- 0:20 / /dev/pts
>
> This has the consequence that the pathname of the parent directory of the
> devpts filesystem mount at /dev/pts is /. So if /dev/ptmx is a bind-mount
> of /dev/pts/ptmx then the /dev/ptmx bind-mount and the devpts mount at
> /dev/pts will end up being located on the same device which is recorded in
> the superblock of their vfsmount. This means the parent directory of the
> /dev/ptmx bind-mount will be /ptmx:
>
> -- -- ---- /ptmx /dev/ptmx
>
> Without the bind-mount resolution patch the kernel will now perform the
> bind-mount escape check directly on /dev/ptmx. The function responsible for
> this is devpts_ptmx_path() which calls pts_path() which in turn calls
> path_parent_directory(). Based on the above explanation,
> path_parent_directory() will yield / as the parent directory for the
> /dev/ptmx bind-mount and not the expected /dev. Thus, the kernel detects
> that /dev/ptmx is escaping its bind-mount and will set /proc/<pid>/fd/<nr>
> to /.
>
> This patch changes the logic to first resolve any bind-mounts. After the
> bind-mounts have been resolved (i.e. we have traced it back to the
> associated devpts mount) devpts_ptmx_path() can be called. In order to
> guarantee correct path generation for the slave file descriptor the kernel
> now requires that a pts directory is found in the parent directory of the
> ptmx bind-mount. This implies that when doing bind-mounts the ptmx
> bind-mount and the devpts mount should have a common parent directory. A
> valid example is:
>
> mount -t devpts devpts /dev/pts
> mount --bind /dev/pts/ptmx /dev/ptmx
>
> an invalid example is:
>
> mount -t devpts devpts /dev/pts
> mount --bind /dev/pts/ptmx /ptmx
>
> This allows us to support:
> - calling open on ptmx devices located inside non-standard devpts mounts:
>   mount -t devpts devpts /mnt
>   master = open("/mnt/ptmx", ...);
>   slave = ioctl(master, TIOCGPTPEER, ...);
> - calling open on ptmx devices located outside the devpts mount with a
>   common ancestor directory:
>   mount -t devpts devpts /dev/pts
>   mount --bind /dev/pts/ptmx /dev/ptmx
>   master = open("/dev/ptmx", ...);
>   slave = ioctl(master, TIOCGPTPEER, ...);
>
> while failing on ptmx devices located outside the devpts mount without a
> common ancestor directory:
>   mount -t devpts devpts /dev/pts
>   mount --bind /dev/pts/ptmx /ptmx
>   master = open("/ptmx", ...);
>   slave = ioctl(master, TIOCGPTPEER, ...);
>
> in which case save path generation cannot be guaranteed.
>
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> Suggested-by: Eric Biederman <ebiederm@...ssion.com>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> ---
> ChangeLog v4->v5:
> * reverse error handling logic to further simplify (Linus)
> ChangeLog v3->v4:
> * simplify if condition (Eric)
> ChangeLog v2->v3:
> * rework logic to account for non-standard devpts mounts such as
>   mount -t devpts devpts /mnt (Christian)
> ChangeLog v1->v2:
> * move removal of if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC)
>   condition to separate patch with non-functional changes (Linus)
> ChangeLog v0->v1:
> * remove
>         /* Has the devpts filesystem already been found? */
>         if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC)
>                         return 0
>   from devpts_ptmx_path() (Eric)
> * check superblock after devpts_ptmx_path() returned (Christian)
> ---
>  fs/devpts/inode.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> index 71b901936113..542364bf923e 100644
> --- a/fs/devpts/inode.c
> +++ b/fs/devpts/inode.c
> @@ -160,21 +160,27 @@ struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
>  	path = filp->f_path;
>  	path_get(&path);
>  
> -	/* Has the devpts filesystem already been found? */
> -	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC)
> +	/* Walk upward while the start point is a bind mount of
> +	 * a single file.
> +	 */
> +	while (path.mnt->mnt_root == path.dentry)
> +		if (follow_up(&path) == 0)
> +			break;
> +
> +	/* devpts_ptmx_path() finds a devpts fs or returns an error. */
> +	if ((path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) ||
> +	    (DEVPTS_SB(path.mnt->mnt_sb) != fsi))
>  		err = devpts_ptmx_path(&path);
>  	dput(path.dentry);
> -	if (err) {
> -		mntput(path.mnt);
> -		return ERR_PTR(err);
> -	}
> +	if (!err) {
> +		if (DEVPTS_SB(path.mnt->mnt_sb) == fsi)
> +			return path.mnt;
>  
> -	if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) {
> -		mntput(path.mnt);
> -		return ERR_PTR(-ENODEV);
> +		err = -ENODEV;
>  	}
>  
> -	return path.mnt;
> +	mntput(path.mnt);
> +	return ERR_PTR(err);
>  }
>  
>  struct pts_fs_info *devpts_acquire(struct file *filp)

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