Messages in this thread | | | From | Andy Lutomirski <> | Date | Sat, 9 Apr 2016 17:44:42 -0700 | Subject | Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup |
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On Sat, Apr 9, 2016 at 5:16 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > On Sat, Apr 9, 2016 at 5:06 PM, H. Peter Anvin <hpa@zytor.com> 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
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