Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

[Andy Lutomirski]

On Sat, Apr 9, 2016 at 5:16 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sat, Apr 9, 2016 at 5:06 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>>
>> Fixing the default permissions is trivial, of course.  The intent from the beginning was to make a ptmx -> pts/ptmx, but user space never did...
>
> That wasn't my point.
>
> Because the permissions have never been usable, I pretty much
> guarantee that no current user space uses /dev/pts/ptmx.
>
> So that node is almost entirely irrelevant. Us fixing the permissions
> at this point isn't going to make it any more relevant, we might as
> well ignore it.
>
> Which all means that the way forward really is to just make /dev/ptmx
> work. It's not going away, and it _is_ fairly easy to fix.
>
> But I don't think the fix should care about permissions - and we might
> as well leave the existing pts/ptmx node with broken permissions.
> Because we've never been actually interested in looking up
> /dev/pts/ptmx - all we actually care about is to look up which devpts
> instance it is.
>
> And that's not about the ptmx node, that's really about the
> mount-point. So the right thing to do - conceptually - is *literally*
> to just say "ok, what is mounted at 'pts'". Note how at no point do we
> want to _open_ anything.
>
> That's why I said that conceptually we could just open /proc/mounts.
> Because *that* is really the operation we care about. We don't care
> about lookup, and we don't care about permissions on the ptmx node.
> Those are completely and utterly irrelevant to what we're actually
> after.
>
> So I think the permission thing is not just extra code with more
> failure points. I think it's conceptually entirely the wrong thing to
> do, and just confuses people into thinking that we're doing something
> that we aren't.

What we *do* want to do, though, is to prevent the following:

Root (or a container manager or whatever) does:

mknod /foo/ptmx c 5 2
chmod 600 /foo/ptmx
chmod 666 /dev/ptmx
mount -t devpts -o newinstance none /foo/pts

Evil user does:

$ unshare -urm
# mount --bind /dev /mnt/foo
# mount --bind /foo/pts /mnt/foo/pts
# open /mnt/foo/ptmx

The issue is that the evil user has the ability to open /mnt/foo/ptmx
(because it's 666), and the relative path 'pts' points to /foo/pts,
which the evil user should *not* be able to access.  IOW, with a naive
implementation, we can match up the ptmx node with the wrong devpts
instance because the evil user unshared their mount namespace and
screwed around.

I don't immediately see how to fix this without playing permission games.

--Andy