linux-kernel - Re: [PATCH] exec: fix the racy usage of fs_struct->in

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Message-ID: <CAGudoHFSzw7KJ-E9qZzfgHs3uoye08po0KJ_cGN_Kumu7ajaBw@mail.gmail.com>
Date: Tue, 25 Mar 2025 12:01:48 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: syzbot <syzbot+1c486d0b62032c82a968@...kaller.appspotmail.com>, 
	brauner@...nel.org, kees@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] exec: fix the racy usage of fs_struct->in_exec

On Tue, Mar 25, 2025 at 11:10 AM Oleg Nesterov <oleg@...hat.com> wrote:
>
> On 03/24, Mateusz Guzik wrote:
> >
> > On Mon, Mar 24, 2025 at 7:28 PM Oleg Nesterov <oleg@...hat.com> wrote:
> > >
> > > So to me it would be better to have the trivial fix for stable,
> > > exactly because it is trivially backportable. Then cleanup/simplify
> > > this logic on top of it.
> >
> > So I got myself a crap testcase with a CLONE_FS'ed task which can
> > execve and sanity-checked that suid is indeed not honored as expected.
>
> So you mean my patch can't fix the problem?

No, I think the patch works.

I am saying the current scheme is avoidably hard to reason about.

>
> > Anyhow, the plan would be to serialize on the bit, synchronized with
> > the current spin lock. copy_fs would call a helper to wait for it to
> > clear, would still bump ->users under the spin lock.
> >
> > This would decouple the handling from cred_mutex and avoid weirdness
> > like clearing the ->in_exec flag when we never set it.
>
> I don't really understand the idea, but as I said I won't argue with
> another solution.
>

I'll try to ship later today so that there will be something concrete
to comment on.

-- 
Mateusz Guzik <mjguzik gmail.com>