linux-kernel - Re: [PATCH v2 4/6] seccomp: handle multiple listeners case

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Message-ID: <CAEivzxe9aRMRK6Ujm17cy_rh3YPJdogvNizBnvKO0WQfQRaNaw@mail.gmail.com>
Date: Thu, 11 Dec 2025 00:18:28 +0100
From: Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To: Tycho Andersen <tycho@...nel.org>
Cc: kees@...nel.org, linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	Andy Lutomirski <luto@...capital.net>, Will Drewry <wad@...omium.org>, Jonathan Corbet <corbet@....net>, 
	Shuah Khan <shuah@...nel.org>, Aleksa Sarai <cyphar@...har.com>, Andrei Vagin <avagin@...il.com>, 
	Christian Brauner <brauner@...nel.org>, Stéphane Graber <stgraber@...raber.org>
Subject: Re: [PATCH v2 4/6] seccomp: handle multiple listeners case

On Thu, Dec 4, 2025 at 4:18 PM Tycho Andersen <tycho@...nel.org> wrote:
>
> On Wed, Dec 03, 2025 at 04:29:49PM +0100, Aleksandr Mikhalitsyn wrote:
> > On Tue, Dec 2, 2025 at 12:52 PM Alexander Mikhalitsyn
> > <aleksandr.mikhalitsyn@...onical.com> wrote:
> > >
> > > If we have more than one listener in the tree and lower listener
> > > wants us to continue syscall (SECCOMP_USER_NOTIF_FLAG_CONTINUE)
> > > we must consult with upper listeners first, otherwise it is a
> > > clear seccomp restrictions bypass scenario.
> > >
> > > Cc: linux-kernel@...r.kernel.org
> > > Cc: bpf@...r.kernel.org
> > > Cc: Kees Cook <kees@...nel.org>
> > > Cc: Andy Lutomirski <luto@...capital.net>
> > > Cc: Will Drewry <wad@...omium.org>
> > > Cc: Jonathan Corbet <corbet@....net>
> > > Cc: Shuah Khan <shuah@...nel.org>
> > > Cc: Aleksa Sarai <cyphar@...har.com>
> > > Cc: Tycho Andersen <tycho@...ho.pizza>
> > > Cc: Andrei Vagin <avagin@...il.com>
> > > Cc: Christian Brauner <brauner@...nel.org>
> > > Cc: Stéphane Graber <stgraber@...raber.org>
> > > Reviewed-by: Tycho Andersen (AMD) <tycho@...nel.org>
> > > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
> > > ---
> > >  kernel/seccomp.c | 33 +++++++++++++++++++++++++++++++--
> > >  1 file changed, 31 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> > > index ded3f6a6430b..262390451ff1 100644
> > > --- a/kernel/seccomp.c
> > > +++ b/kernel/seccomp.c
> > > @@ -448,8 +448,21 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
> > >
> > >                 if (ACTION_ONLY(cur_ret) < ACTION_ONLY(ret)) {
> > >                         ret = cur_ret;
> > > +                       /*
> > > +                        * No matter what we had before in matches->filters[],
> > > +                        * we need to overwrite it, because current action is more
> > > +                        * restrictive than any previous one.
> > > +                        */
> > >                         matches->n = 1;
> > >                         matches->filters[0] = f;
> > > +               } else if ((ACTION_ONLY(cur_ret) == ACTION_ONLY(ret)) &&
> > > +                           ACTION_ONLY(cur_ret) == SECCOMP_RET_USER_NOTIF) {
> >
> > My bad. We also have to check f->notif in there like that:
>

Hi Tycho,

sorry for the delay with a reply.

> For my own education: why is that? Shouldn't
> seccomp_do_user_notification() be smart enough to catch this case (and
> indeed, there is a TOCTOU if you do it here?)?

seccomp_do_user_notification() is smart enough to handle the case when
a listener file descriptor was closed,
but a tricky part here is that SECCOMP_RET_USER_NOTIF can be (legally)
returned by the seccomp filter
program even when there was no listener at all.

Then, as nothing prevents you from loading a program like:
    BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_USER_NOTIF)
with
     seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog) // << no
SECCOMP_FILTER_FLAG_NEW_LISTENER flag

we can easily do OOB write in matches->filters[] array, because our
limitation with 8 elements only works for those who
set the SECCOMP_FILTER_FLAG_NEW_LISTENER flag.

Actually, I decided to get rid of this array in the next version of patchset.

Hope to virtually meet you at LPC soon! ;)

Kind regards,
Alex

>
> Thanks,
>
> Tycho