Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755467Ab3JaTJc (ORCPT ); Thu, 31 Oct 2013 15:09:32 -0400 Received: from mail-vc0-f179.google.com ([209.85.220.179]:40635 "EHLO mail-vc0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753436Ab3JaTJ3 convert rfc822-to-8bit (ORCPT ); Thu, 31 Oct 2013 15:09:29 -0400 MIME-Version: 1.0 In-Reply-To: <5068709.xK6KPeP0iH@jlieb-e6410> References: <1381960919-4542-1-git-send-email-jlieb@panasas.com> <8761snb44l.fsf@xmission.com> <5068709.xK6KPeP0iH@jlieb-e6410> From: Andy Lutomirski Date: Thu, 31 Oct 2013 12:09:08 -0700 Message-ID: Subject: Re: Re: [PATCH 1/3] switch_creds: Syscall to switch creds for file server ops To: Jim Lieb Cc: Linux FS Devel , "Eric W. Biederman" , Al Viro , "Theodore Ts'o" , "linux-kernel@vger.kernel.org" , bfields@redhat.com, Jeff Layton Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 9421 Lines: 204 On Thu, Oct 24, 2013 at 1:24 PM, Jim Lieb wrote: > On Thursday, October 24, 2013 12:28:15 Andy Lutomirski wrote: >> On Wed, Oct 23, 2013 at 10:59 PM, Eric W. Biederman >> >> wrote: >> > Andy Lutomirski writes: >> >> On 10/16/2013 08:52 PM, Eric W. Biederman wrote: >> >>> Al Viro writes: >> >>>> On Wed, Oct 16, 2013 at 06:18:16PM -0700, Eric W. Biederman wrote: >> >>>>> That doesn't look bad but it does need capable(CAP_SETUID) && >> >>>>> capable(CAP_SETGID) or possibly something a little more refined. >> >>>> >> >>>> D'oh >> >>>> >> >>>>> I don't think we want file descriptor passing to all of a sudden >> >>>>> become >> >>>>> a grant of privilege, beyond what the passed fd can do. >> >>>> >> >>>> Definitely. And an extra ) to make it compile wouldn't hurt either... >> >>> >> >>> There also appears to need to be a check that we don't gain any >> >>> capabilities. >> >>> >> >>> We also need a check so that you don't gain any capabilities, and >> >>> possibly a few other things. >> >> >> >> Why? I like the user_ns part, but I'm not immediately seeing the issue >> >> with capabilities. >> > >> > My reasoning was instead of making this syscall as generic as possible >> > start it out by only allowing the cases Jim cares about and working with >> > a model where you can't gain any permissions you couldn't gain >> > otherwise. >> > >> > Although the fd -1 trick to revert to your other existing cred seems >> > reasonable. >> > >> >>> So I suspect we want a check something like: >> >>> >> >>> if ((new_cred->securebits != current_cred->securebits) || >> >>> >> >>> (new_cred->cap_inheritable != current_cred->cap_inheritable) || >> >>> (new_cred->cap_permitted != current_cred->cap_permitted) || >> >>> (new_cred->cap_effective != current_cred->cap_effective) || >> >>> (new_cred->cap_bset != current_cred->cap_bset) || >> >>> (new_cred->jit_keyring != current_cred->jit_keyring) || >> >>> (new_cred->session_keyring != current_cred->session_keyring) || >> >>> (new_cred->process_keyring != current_cred->process_keyring) || >> >>> (new_cred->thread_keyring != current_cred->thread_keyring) || >> >>> (new_cred->request_keyring != current_cred->request_keyring) || >> >>> (new_cred->security != current_cred->security) || >> >>> (new_cred->user_ns != current_cred->user_ns)) { >> >>> >> >>> return -EPERM; >> >>> >> >>> } >> >> >> >> I *really* don't like the idea of being able to use any old file >> >> descriptor. I barely care what rights the caller needs to have to >> >> invoke this -- if you're going to pass an fd that grants a capability >> >> (in the non-Linux sense of the work), please make sure that the sender >> >> actually wants that behavior. >> >> >> >> IOW, have a syscall to generate a special fd for this purpose. It's >> >> only a couple lines of code, and I think we'll really regret it if we >> >> fsck this up. >> >> >> >> (I will take it as a personal challenge to find at least one exploitable >> >> privilege escalation in this if an arbitrary fd works.) >> > >> > If you can't switch to a uid or a gid you couldn't switch to otherwise >> > then the worst that can happen is an information leak. And information >> > leaks are rarely directly exploitable. >> >> Here's the attack: >> >> Suppose there's a daemon that uses this in conjunction with >> SCM_RIGHTS. The daemon is effectively root (under the current >> proposal, it has to be). So a client connects, sends a credential fd, >> and the daemon impersonates those credentials. >> >> Now a malicious user obtains *any* fd opened by root. It sends that >> fd to the daemon. The daemon then impersonates root. We lose. (It >> can't possibly be hard to obtain an fd with highly privileged f_cred >> -- I bet that most console programs have stdin like that, for example. >> There are probably many setuid programs that will happily open >> /dev/null for you, too.) > > In my reply to Eric, I note that I need to add a check that the fd passed is > one from switch_creds. With that test, not any fd will do and the one that > does has only been able to set fsuid, fsgid, altgroups, and reduced (the nfsd > set) caps. They can do no more damage than what the original switch_creds > allowed. The any fd by root no longer applies so use doesn't get much (no > escalation). > >> >> >> Also... real_cred looks confusing. AFAICS it is used *only* for knfsd >> >> and faccessat. That is, current userspace can't see it. But now you'll >> >> expose various oddities. For example, AFAICS a capability-less process >> >> that's a userns owner can always use setuid. This will *overwrite* >> >> real_cred. Then you're screwed, especially if this happens by >> >> accident. >> > >> > And doing in userland what faccessat, and knfsd do in the kernel is >> > exactly what is desired here. But maybe there are issues with that. >> > >> >> That being said, Windows has had functions like this for a long time. >> >> Processes have a primary token and possibly an impersonation token. Any >> >> process can call ImpersonateLoggedOnUser (no privilege required) to >> >> impersonate the credentials of a token (which is special kind of fd). >> >> Similarly, any process can call RevertToSelf to undo it. >> >> >> >> Is there any actual problem with allowing completely unprivileged tasks >> >> to switch to one of these magic cred fds? That would avoid needing a >> >> "revert" operation. >> > >> > If the permission model is this switching of credentials doesn't get you >> > anything you couldn't get some other way. That would seem to totally >> > rules out unprivileged processes switching these things. >> >> IMO, there are two reasonable models that involve fds carrying some >> kind of credential. >> >> 1. The fd actually carries the ability to use the credentials. You >> need to be very careful whom you send these to. The security >> implications are obvious (which is good) and the receiver doesn't need >> privilege (which is good, as long as the receiver is careful). > > I test for caps. I think using switch_creds in all forms should require > privs. I thought of the revert case and although it does simply return to > "real" creds, you still need CAP_SETUID and CAP_SETGID to do it. This means, > of course, if you have really messed up things, you may not be able to get > back home which, although a bad thing for the process, is a good thing for the > system as a whole. > >> >> 2. The fd is for identification only. But this means that the fd >> carries the ability to identify as a user. So you *still* have to be >> very careful about whom you send it to. What you need is an operation >> that allows you to identify using the fd without transitively granting >> the recipient the same ability. On networks, this is done by signing >> some kind of challenge. The kernel could work the same way, or there >> could be a new CMSG_IDENTITY that you need an identity fd to send but >> that does not copy that fd to the recipient. > > I am not sure I understand this. CMSG only applies to UNIX_DOMAIN sockets > which means that the switch_creds fd test still applies here. It is > identification but only for within the same kernel. As for namespaces, the > translation was done when the creds fd was created. I suppose if it was > passed across namespace boundaries there could be a problem but what is in the > creds structure is the translated fduid,fsgid etc., not the untranslated one. > We are still doing access checks and quota stuff with the translated creds. If > one namespace has "fred" as uid 52 and another has "mary" as 52, the quota > will only be credited to whoever "fred" really is only if "fred" gets to be > "mary" in the second alternate universe... I'm not talking about namespaces here -- I think we're not really on the same page. Can you describe what you're envisioning doing with these fds? >> >> >> Another note: I think that there may be issues if the creator of a token >> >> has no_new_privs set and the user doesn't. Imagine a daemon that >> >> accepts one of these fds, impersonates it, and calls exec. This could >> >> be used to escape from no_new_privs land. >> > >> > Which is why I was suggesting that we don't allow changing any field in >> > the cred except for uids and gids. >> >> If the daemon impersonates and execs, we still lose. > > I answered this. You only get to impersonate for purposes of file access with > reduced caps to prevent you from being root as well. Also, since these are > O_CLOEXEC and switch_creds "magic" fds, this can't happen because the fd is > gone post-exec. If a no_new_privs process authenticates to a daemon and that daemon execs, then there's now a dumpable, ptraceable, non-no-new-privs process running as the uid/gid of the no_new_privs process. This may be dangerous. > >> >> >> --Andy > > -- > Jim Lieb > Linux Systems Engineer > Panasas Inc. > > "If ease of use was the only requirement, we would all be riding tricycles" > - Douglas Engelbart 1925?2013 -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/