linux-kernel - Re: [PATCH RESEND v5 0/5] namei: vfs flags to restrict path resolution

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Date:   Tue, 26 Mar 2019 00:04:29 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian@...uner.io>,
        David Drysdale <drysdale@...gle.com>,
        Tycho Andersen <tycho@...ho.ws>,
        Kees Cook <keescook@...omium.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>, Aleksa Sarai <asarai@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH RESEND v5 0/5] namei: vfs flags to restrict path
 resolution

On 2019-03-21, Andy Lutomirski <luto@...nel.org> wrote:
> On Wed, Mar 20, 2019 at 7:38 AM Aleksa Sarai <cyphar@...har.com> wrote:
> > Now that the holiday break is over, it's time to re-send this patch
> > series (with a few additions, due to new information we got from
> > CVE-2019-5736 -- which this patchset mostly protected against but had
> > some holes with regards to #!-style scripts).
> 
> I generally like this, but, as Linus pointed out, it will be
> unfortunate if application authors see this as just another
> non-portable weird Linux API and don't use it.  Would it be worthwhile
> to put some thought into making it an API that other OSes might be
> willing to implement?  As it stands, the openat(2) flags are getting
> rather crazy in this patch set.
> 
> Aleksa had a resolveat(2) proposal that really didn't seem too bad.

I agree having a bunch of O_* flags for resolution feels quite ugly (and
crazy) -- but the last time I pitched resolveat(2) the reaction was
lukewarm to say the least. It's basically an O_PATHv2 and I don't know
how popular that suggestion might be. I wouldn't mind pitching it again
though (now that I have a better idea of how to handle some of the UX
worries I had).

But, if we have a resolveat(2) we'll need to add O_EMPTYPATH support for
openat(2) so that you can open scoped paths without using the
/proc/self/fd/$n trick. However, you run into reopening permission
issues (as we saw in CVE-2019-5736 and countless other CVEs). There
might be a solution for this which Andy and I have talked about
off-list.

There is also the problem of the execveat(2) attack -- which is dealt
with in patch 5 of this series. In order to scope #!-script resolution
we need to have AT_* flags for the same resolution restriction
(defeating the point of a resolveat(2) slightly).

There is an argument that we shouldn't need AT_THIS_ROOT or AT_BENEATH
support (because ideally you should be doing execve(2) in a pivot_root
anyway) but AT_NO_MAGICLINKS is pretty important -- since it allows you
to block the circumvention mm->dumpable in certain situations (as we saw
in CVE-2019-5736).

The solution Andy and I have discussed is a way to make fd-reopening (a
seemingly little-known trick outside of container runtimes) much safer
such that we don't need mitigations like AT_NO_MAGICLINKS on
execveat(2). If we assume that idea works out (I'm still trying to get a
working patch for that idea) then resolveat(2) would be sufficient and
we don't need AT_* flags on execveat(2).

tl;dr: I think resolveat(2) is much nicer, and assuming it's not an
	   unpopular idea I think we should go for it. But there are several
	   other threads of discussion that we might want to have first
	   (such as how to improve the fd-reopening design so it's safer
	   before we expose it to everyone).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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