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

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Date:   Fri, 10 May 2019 22:10:32 +0200
From:   Jann Horn <jannh@...gle.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Aleksa Sarai <cyphar@...har.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>,
        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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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 Tue, May 07, 2019 at 07:38:58PM -0500, Eric W. Biederman wrote:
> Jann Horn <jannh@...gle.com> writes:
> > In my opinion, CVE-2019-5736 points out two different problems:
> >
> > The big problem: The __ptrace_may_access() logic has a special-case
> > short-circuit for "introspection" that you can't opt out of;
> 
> Once upon a time in a galaxy far far away I fixed a bug where we missing
> ptrace_may_access checks on various proc files and systems using selinux
> stopped working.  At the time selinux did not allow ptrace like access
> to yourself.  The "introspection" special case was the quick and simple
> work-around.
> 
> There is nothing fundamental in having the "introspection" special case
> except that various lsms have probably grown to depend upon it being
> there.  I expect without difficulty we could move the check down
> into the various lsms.  Which would get that check out of the core
> kernel code.

Oh, if that's an option, that would be great, I think.


But this means, for example, that a non-root, non-dumpable process can't
open /proc/self/maps anymore, or open /proc/self/fd/*, and things like
that, without making itself dumpable. I would be surprised if there is
no code out there that relies on that.

>From what I can tell, without the introspection special case,
introspection would fail in the following cases (assuming that the
process is not capable and isn't using sys_setfs[ug]id()):

 - ruid/euid/suid are not all the same
 - rgid/egid/sgid are not all the same
 - process is not dumpable

I think that there probably should be some way for a non-dumpable
process to look at its own procfs entries? If we could start from a
clean slate, I'd propose an opt-in flag to openat() for that, but
since we don't have a clean slate, I'd be afraid of breaking things
with that. But maybe I'm just being overly careful here?