On Mon, Apr 11, 2016 at 1:12 PM, Andy Lutomirski <l...@amacapital.net> wrote: > On Sat, Apr 9, 2016 at 6:27 PM, Linus Torvalds > <torva...@linux-foundation.org> wrote: >> >> On Apr 9, 2016 5:45 PM, "Andy Lutomirski" <l...@amacapital.net> wrote: >>> >>> >>> What we *do* want to do, though, is to prevent the following: >> >> I don't see the point. Why do you bring up this insane scenario that nobody >> can possibly care about? >> >> So you actually have any reason to believe somebody does that? >> >> I already asked about that earlier, and the silence was deafening. > > I have no idea, but I'm generally uncomfortable with magical things > that bypass normal security policy. > > That being said, here's an idea for fixing this, at least in the long > run. Add a new devpts mount option "no_ptmx_redirect" that turns off > this behavior for the super in question. That is, opening /dev/ptmx > if "pts/ptmx" points to something with no_ptmx_redirect set will fail. > Distros shipping new kernels could be encouraged to (finally!) make > /dev/ptmx a symlink and set this option. > > We just might be able to get away with spelling that option "newinstance".
Linus, you said that people who want to protect their pts should deny execute. So I set it up: # ls -l total 0 crw-------. 1 root root 5, 2 Apr 12 10:38 ptmx drwx------. 2 root root 0 Apr 2 11:35 pts $ unshare -urm # ls -l total 0 crw-------. 1 nfsnobody nfsnobody 5, 2 Apr 12 10:38 ptmx drwx------. 2 nfsnobody nfsnobody 0 Apr 2 11:35 pts # mount --bind /dev/ptmx ptmx # ls -l total 0 crw-rw-rw-. 1 nfsnobody nfsnobody 5, 2 Apr 12 10:42 ptmx drwx------. 2 nfsnobody nfsnobody 0 Apr 2 11:35 pts And there goes your protection. So the whole /dev directory would have to deny execute to protect against this. But I think that gating this on mount options might be fine. If devpts is mounted with newinstance, then /dev/ptmx *already doesn't work for it*, right? So can we just say that the magic ptmx -> pts/ptmx redirect doesn't work if the pts filesystem in question is mounted with newinstance? --Andy