linux-kernel - Re: [PATCH 1/2] fs/exec: Explicitly unshare fs

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Date:   Thu, 6 Oct 2022 17:35:37 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Christian Brauner <brauner@...nel.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Jorge Merlino <jorge.merlino@...onical.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        John Johansen <john.johansen@...onical.com>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        Richard Haines <richard_c_haines@...nternet.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Xin Long <lucien.xin@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Todd Kjos <tkjos@...gle.com>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        Prashanth Prahlad <pprahlad@...hat.com>,
        Micah Morton <mortonm@...omium.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Andrei Vagin <avagin@...il.com>, linux-kernel@...r.kernel.org,
        apparmor@...ts.ubuntu.com, linux-security-module@...r.kernel.org,
        selinux@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/2] fs/exec: Explicitly unshare fs_struct on exec

On Thu, Oct 6, 2022 at 5:25 PM Kees Cook <keescook@...omium.org> wrote:
> On October 6, 2022 7:13:37 AM PDT, Jann Horn <jannh@...gle.com> wrote:
> >On Thu, Oct 6, 2022 at 11:05 AM Christian Brauner <brauner@...nel.org> wrote:
> >> On Thu, Oct 06, 2022 at 01:27:34AM -0700, Kees Cook wrote:
> >> > The check_unsafe_exec() counting of n_fs would not add up under a heavily
> >> > threaded process trying to perform a suid exec, causing the suid portion
> >> > to fail. This counting error appears to be unneeded, but to catch any
> >> > possible conditions, explicitly unshare fs_struct on exec, if it ends up
> >>
> >> Isn't this a potential uapi break? Afaict, before this change a call to
> >> clone{3}(CLONE_FS) followed by an exec in the child would have the
> >> parent and child share fs information. So if the child e.g., changes the
> >> working directory post exec it would also affect the parent. But after
> >> this change here this would no longer be true. So a child changing a
> >> workding directoro would not affect the parent anymore. IOW, an exec is
> >> accompanied by an unshare(CLONE_FS). Might still be worth trying ofc but
> >> it seems like a non-trivial uapi change but there might be few users
> >> that do clone{3}(CLONE_FS) followed by an exec.
> >
> >I believe the following code in Chromium explicitly relies on this
> >behavior, but I'm not sure whether this code is in active use anymore:
> >
> >https://source.chromium.org/chromium/chromium/src/+/main:sandbox/linux/suid/sandbox.c;l=101?q=CLONE_FS&sq=&ss=chromium
>
> Oh yes. I think I had tried to forget this existed. Ugh. Okay, so back to the drawing board, I guess. The counting will need to be fixed...
>
> It's possible we can move the counting after dethread -- it seems the early count was just to avoid setting flags after the point of no return, but it's not an error condition...

Random idea that I haven't thought about a lot:

One approach might be to not do it by counting, but instead have a
flag on the fs_struct that we set when someone does a clone() with
CLONE_FS but without CLONE_THREAD? Then we'd end up with the following
possible states for fs_struct:

 - single-process, normal
 - single-process, pending execve past check_unsafe_exec() (prevent
concurrent CLONE_FS)
 - shared between processes

The slight difference from the old semantics would be that once you've
used CLONE_FS without CLONE_THREAD, you can never do setuid execve()
from your current process again (without calling unshare()), even if
the child disappears in the meantime. I think that might be an
acceptably small UAPI break.