linux-kernel - Re: [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters

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Message-ID: <20190511173113.qhqmv5q5f74povix@yavin>
Date:   Sun, 12 May 2019 03:31:13 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Jann Horn <jannh@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Christian Brauner <christian@...uner.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>, Aleksa Sarai <asarai@...e.de>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters

On 2019-05-11, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Sat, May 11, 2019 at 1:21 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Notice? None of the real problems are about execve or would be solved
> > by any spawn API. You just think that because you've apparently been
> > talking to too many MS people that think fork (and thus indirectly
> > execve()) is bad process management.
> 
> Side note: a good policy has been (and remains) to make suid binaries
> not be dynamically linked. And in the absence of that, the dynamic
> linker at least resets the library path when it notices itself being
> dynamic, and it certainly doesn't inherit any open flags from the
> non-trusted environment.
> 
> And by the same logic, a suid interpreter must *definitely* should not
> inherit any execve() flags from the non-trusted environment. So I
> think Aleksa's patch to use the passed-in open flags is *exactly* the
> wrong thing to do for security reasons. It doesn't close holes, it
> opens them.

Yup, I've dropped the patch for the next version. (To be honest, I'm not
sure why I included any of the other flags -- the only one that would've
been necessary to deal with CVE-2019-5736 was AT_NO_MAGICLINKS.)

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

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