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

[Christian Brauner]

On Wed, Mar 07, 2018 at 01:30:52PM -0600, Eric W. Biederman wrote:
> 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.

It's at least a regression since this used to work before. :)

> 
> > 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.

It's quite important for containers. The problem is that we can't (yet)
mknod() in a user namespace and making /dev/ptmx a symlink to
/dev/pts/ptmx will cause issues when used together with path-based LSMs
like AppArmor so a bind-mount is the only reliable option.

> 
> The code can be improved by doing something like:

Right, what do you think about Linus suggestion? I'm happy to look into
it.

Christian

> 
> 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