On Tue, Apr 12, 2016 at 10:44 AM, Andy Lutomirski <l...@amacapital.net> wrote: > 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
No you didn't. You're root, and you still have access to /dev/ptmx. > And there goes your protection. So the whole /dev directory would > have to deny execute to protect against this. Exactly. That's what I'm saying. If you want your ptmx to be private, you need to make your /dev private. Now, you can avoid the other attack that was talked about (which involved bind-mounting the pts/ directory somewhere else) by making just the pts/ directory non-execute, because afaik bind mount requires the ability to do the lookup. > 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? No, the problem that started this whole discussion is that (a) newinstance should go the f*ck away, because this whole duality is broken. (b) people wanted single instances and we couldn't even enable default kernel support for DEVPTS_MULTIPLE_INSTANCES, because multiple instances just don't work with /dev/ptmx. So what I want to happen is to "just make /dev/ptmx work". Get rid of the broken "single instance" crap. The only reason it exists is exactly because /dev/ptmx does not work. I think the current situation is completely and utterly broken. We should never have done what we did. I want to *fix* the kernel, not add random new magic crap. And I think we _can_ fix the kernel. Not add new mount options that people already don't use (because they are broken for the normal situation). Linus