Re: Invalid /proc/<pid>/fd/{0,1,2} symlinks with TIOCGPTPEER

[Eric W. Biederman]

Christian Brauner <christian.brauner@xxxxxxxxxxxxx> writes:

> Hey,
>
> We discovered a potential bug in the devpts implementation via
> TIOCGPTPEER ioctl()s today. We've tackled a similar problem already in:
>
> commit 311fc65c9fb9c966bca8e6f3ff8132ce57344ab9
> Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Date:   Thu Aug 24 15:13:29 2017 -0500
>
>     pty: Repair TIOCGPTPEER
>
> Most libcs will *still* look at /dev/ptmx when opening the master fd of
> pty device. Usually, /dev/ptmx will nowadays be either a symlink to
> /dev/pts/ptmx or it will be a second device node with permissions 666
> whereas /dev/pts/ptmx will usually have permissions 000. Afaik, we've
> also always supported making /dev/ptmx a bind-mount to /dev/pts/ptmx or
> at least I haven't observed any issues with this so far and it's
> something fairly common in containers. So in short, it should be legal
> to do:
>
> mount --bind /dev/pts/ptmx /dev/ptmx
> chmod 666 /dev/ptmx
>
> However, for any libc implementation or program that uses TIOCGPTPEER
> the /proc/<pid>/fd/{0,1,2} symlinks are broken (currently affects at
> least glibc 2.27) with bind-mounts of /dev/pts/ptmx to /dev/ptmx. A
> quick reproducer is:
>
> unshare --mount
> mount --bind /dev/pts/ptmx /dev/ptmx
> chmod 666 /dev/ptmx
> script
> ls -al /proc/self/fd/0
>
> Let's assume the slave device index I received was 5 then I would expect to
> see:
>
> ls -al /proc/self/fd/0
> lrwx------ 1 chb chb 64 Mar  7 16:41 /proc/self/fd/0 -> /dev/pts/5
>
> But what I actually see is:
>
> ls -al /proc/self/fd/0
> lrwx------ 1 chb chb 64 Mar  7 16:41 /proc/self/fd/0 -> /
>
> I think the explanation for this is fairly straightforward. When
> userspace does:
>
> master = open("/dev/ptmx", O_RDWR | O_NOCTTY);
> slave = ioctl(master, TIOCGPTPEER, O_RDWR | O_NOCTTY);
>
> and /dev/ptmx is a bind-mount of /dev/pts/ptmx looking up the root mount
> of the dentry for the slave it appears to the kernel as if the dentry is
> escaping it's bind-mount:
>
> ââ/dev        udev          devtmpfs   rw,nosuid,relatime,size=4001260k,nr_inodes=1000315,mode=755
> â ââ/dev/pts  devpts        devpts     rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000
> â ââ/dev/ptmx devpts[/ptmx] devpts     rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000
>
> since the root mount of the dentry is /dev/pts but the root mount of
> /dev/ptmx is /dev if I'm correct so similar to what Linus pointed out in
> a previous discussion (see [1]) before. So we still record the "wrong"
> vfsmount when /dev/ptmx is a bind-mount and then hit the problem when we
> call devpts_mntget() in drivers/tty/pty.c.

I think your analysis of why we return / is correct. If the root of the
mount is a file (aka /dev/pts/ptmx).  Then any other file will on that
mount will not be under the root of the mount, and will be displayed
as '/'.  Because we have in fact escaped the root of the mount.

I think this is more of a quality of implementation issue more than a
bug per se.

> So I thought about this and - in case my analysis is correct - the
> solution didn't seem obvious to me as a bind-mount has no concept of
> what it's "parent" is (Which in this case should be the devpts mount at
> /dev/pts.).

We might be able to improve the quality of the implementation, by
noticing this case early (sb->s_root != mnt->mnt_root) and using the
same tricks on /dev/pts/ptmx as we do on /dev/ptmx.  That is looking
in ../pts and see if the filesystem we want is there.

It would be a wee bit tricky but doable.  The practical question becomes
what breaks and what makes it worth maintaining such a mechanism.

I don't remember how important it is to have a valid path in proc.  So
I won't comment on how important it is to improve the quality of
the implementation.

The code can be improved by doing something like:

static int devpts_ptmx_pts_path(struct path *path)
{
	struct super_block *sb;
	int err;

	/* Is a devpts filesystem at "pts" in the same directory? */
	err = path_pts(path);
	if (err)
		return err;

	/* Is the path the root of a devpts filesystem? */
	sb = path->mnt->mnt_sb;
	if ((sb->s_magic != DEVPTS_SUPER_MAGIC) ||
	    (path->mnt->mnt_root != sb->s_root))
		return -ENODEV;

	return 0;
}

....
	if ((DEVPTS_SB(path.mnt->mnt_sb) == fsi) &&
	    (path.mnt->mnt_root == fsi->ptmx_dentry)) {
		/* While the start point is a bind mount of single file
                 * walk upwards.
                 */
            	while ((path.mnt->mnt_root == path.dentry) && follow_up(&path))
			;
		if (devpts_ptmx_pts_path(&path) == 0) {
                	dput(path.dentry);
                   	return path.mnt;     
                }
                /* No luck fall through to the old code */
                path_put(path);
                path = filp->f_path;
                path_get(&path);
        }

The fall through vs fail would be a judgement on how important it is to
have a useable path in proc for TIOCPTPEER.

Eric