Date: Fri, 4 Oct 2013 19:34:08 +0100 From: Andy Lutomirski <luto@...capital.net> To: Djalal Harouni <tixxdz@...ndz.org> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, Kees Cook <keescook@...omium.org>, Al Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Ingo Molnar <mingo@...nel.org>, "Serge E. Hallyn" <serge.hallyn@...ntu.com>, Cyrill Gorcunov <gorcunov@...nvz.org>, David Rientjes <rientjes@...gle.com>, LKML <linux-kernel@...r.kernel.org>, Linux FS Devel <linux-fsdevel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Djalal Harouni <tixxdz@...il.com> Subject: Re: [PATCH v2 2/9] procfs: add proc_allow_access() to check if file's opener may access task On Fri, Oct 4, 2013 at 7:23 PM, Djalal Harouni <tixxdz@...ndz.org> wrote: > On Fri, Oct 04, 2013 at 04:40:01PM +0100, Andy Lutomirski wrote: >> On Fri, Oct 4, 2013 at 9:59 AM, Djalal Harouni <tixxdz@...ndz.org> wrote: >> > On Thu, Oct 03, 2013 at 02:09:55PM -0700, Andy Lutomirski wrote: >> >> On Thu, Oct 3, 2013 at 1:13 PM, Djalal Harouni <tixxdz@...ndz.org> wrote: >> >> > On Thu, Oct 03, 2013 at 12:37:49PM -0700, Andy Lutomirski wrote: >> >> >> On Thu, Oct 3, 2013 at 12:29 PM, Djalal Harouni <tixxdz@...ndz.org> wrote: >> > The current patches block and protect the current attacks correctly, >> > without overhead. >> > >> > Example: >> > proc_uid_map_write() >> > -> map_write() >> > -> file_ns_capable() >> > -> security_capable(file->f_cred, ns, cap) >> > >> > file_ns_capable() added in commit 935d8aabd4331 by Linus >> > Add file_ns_capable() helper function for open-time capability checking >> > >> > That also goes for commit 6708075f104c3c9b0 by Eric, >> > userns: Don't let unprivileged users trick privileged users into setting >> > the id_map >> > >> > The proc_allow_access() function that I've proposed has the same logic >> > of file_ns_capable(), We can even put file_ns_capable() inside >> > proc_allow_access(). We'll add support of security_capable_noaudit() >> > inside file_ns_capable() and proc_allow_access() will be much more >> > better. >> > >> > file_ns_capable() checks where a capability was there, >> > proc_allow_access() checks where they have same uid + if capability was >> > there. >> > >> >> please either fix it, stop using f_cred, or explain why it it's okay >> >> despite not invoking LSM in the expected way. >> > I've already explained it. >> > >> > LSM is handled by ptrace_may_access() which should be called during >> > ->open() to handle f_cred, and during ->read() to handle current's cred. >> >> This is getting tiresome. This patch (2/9) has my NAK. The other >> patches depend on it, so I will not ack them. (The maintainers may or >> may not care about my NAK -- that's their business.) > Yeh tiresome, especially for fixing some leaks that affect the kernel > and ASLR for a long time now... > > Ok response below: > >> Your code is *wrong* for even the simple case of /proc/*/syscall. Consider: >> >> Start with two processes, a and b, both normal tasks started by an >> unprivileged user. Process a opens /proc/<b's pid>/syscall. All >> checks pass. Process b execs a setcap'd binary. So b's uid and gid >> do not change. >> >> Then process a redirects stdin to that existing /proc/<b's pid>/stack > You mean /proc/<b's pid>/syscall, ok let see > >> fd. Here's the bug in your patch: process a can *still* read that fd. >> Why? Because *you're not checking that a's capabilities are a >> superset of b's*. That code lives in the LSM infrastructure. You > Please see with me: Sorry, I described the obviously broken scenario incorrectly. Your patch breaks (in the absence of things like selinux) if a exec something setuid root. [...] > > I did the check in the proc_same_open_cred() function: > return (uid_eq(fcred->uid, cred->uid) && > gid_eq(fcred->gid, cred->gid) && > cap_issubset(cred->cap_permitted, fcred->cap_permitted)); Which has nothing to do with anything. If that check fails, you're just going on to a different, WRONG check/. > > Check if this is the same uid/gid and the capabilities superset! > > But in the proc_allow_access() the capabilities superset is missing. > > > So to fix it: > 1) if proc_same_open_cred() detects that cred have changed between > ->open() and ->read() then abort, return zero, the ->read(),write()... IMO yuck. > > > 2) Improve the proc_allow_access() check by: > if this is the same user namespace then check uid/gid of f_cred on > target cred task, and the capabilities superset: > cap_issubset(tcred->cap_permitted, fcred->cap_permitted)); > > If it fails let security_capable() or file_ns_capable() do its magic. > NAK. You need to actually call the LSM. What if the reason to fail the request isn't that the writer gained capabilities -- what if the writer's selinux label changed? > > > The capabilities superset check are available, they are not LSM > specific. NO. This is getting ridiculous. You can't just re-implement what the commoncap code does because you don't want to call the LSM hook. You need to call the LSM hook. > >> need to call it if you want to keep the general approach you're >> trying. You can't fix this just by checking for CAP_PTRACE, because >> then you'll break SELinux. > IIUC, then SELinux is already broken! > > You do you realize that these patches are adding a bench of > ptrace_may_access() during ->open() to give LSM a chance to inspect > f_cred and other things. Yes. But they're messy, they open-code incorrect checks, and they are sufficiently incomplete as fixes for the vulnerability that you're better off not doing them, because they'll just muddy the waters. --Andy > > >> This is messy, and it's why I think that you'd be better off doing >> this by revoking the fd on exec instead. > As I've said I'm not against revoke. If it was there I would use it! > > Not implemented! and perhaps it will be complex... > > > Please respond, and perhaps you should reconsider your NAK! since you > were involved in the patches that were committed, and used the > file->f_cred approach Yes, I was. That particular thing is fine, because there's no LSM hook involved in the first case. If anyone ever adds one, they'll have to invoke it in the appropriate places. --Andy
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.