linux-kernel - Re: [PATCH] syscalls: Document OCI seccomp filter interactions & workaround

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Message-ID: <CAG48ez3fHajON65uuKNx7Q+52X=41-C7=mub=XVmBWSUtBtnpA@mail.gmail.com>
Date:   Tue, 24 Nov 2020 18:47:24 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, Mark Wielaard <mark@...mp.org>,
        Florian Weimer <fweimer@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Linux API <linux-api@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        dev@...ncontainers.org, Jonathan Corbet <corbet@....net>,
        "Carlos O'Donell" <carlos@...hat.com>
Subject: Re: [PATCH] syscalls: Document OCI seccomp filter interactions & workaround

On Tue, Nov 24, 2020 at 6:44 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> On Tue, Nov 24, 2020 at 06:30:28PM +0100, Jann Horn wrote:
> > On Tue, Nov 24, 2020 at 6:15 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> > > On Tue, Nov 24, 2020 at 06:06:38PM +0100, Jann Horn wrote:
> > > > +seccomp maintainers/reviewers
> > > > [thread context is at
> > > > https://lore.kernel.org/linux-api/87lfer2c0b.fsf@oldenburg2.str.redhat.com/
> > > > ]
> > > >
> > > > On Tue, Nov 24, 2020 at 5:49 PM Christoph Hellwig <hch@...radead.org> wrote:
> > > > > On Tue, Nov 24, 2020 at 03:08:05PM +0100, Mark Wielaard wrote:
> > > > > > For valgrind the issue is statx which we try to use before falling back
> > > > > > to stat64, fstatat or stat (depending on architecture, not all define
> > > > > > all of these). The problem with these fallbacks is that under some
> > > > > > containers (libseccomp versions) they might return EPERM instead of
> > > > > > ENOSYS. This causes really obscure errors that are really hard to
> > > > > > diagnose.
> > > > >
> > > > > So find a way to detect these completely broken container run times
> > > > > and refuse to run under them at all.  After all they've decided to
> > > > > deliberately break the syscall ABI.  (and yes, we gave the the rope
> > > > > to do that with seccomp :().
> > > >
> > > > FWIW, if the consensus is that seccomp filters that return -EPERM by
> > > > default are categorically wrong, I think it should be fairly easy to
> > > > add a check to the seccomp core that detects whether the installed
> > > > filter returns EPERM for some fixed unused syscall number and, if so,
> > > > prints a warning to dmesg or something along those lines...
> > >
> > > Why?  seccomp is saying "this syscall is not permitted", so -EPERM seems
> > > like the correct error to provide here.  It's not -ENOSYS as the syscall
> > > is present.
> > >
> > > As everyone knows, there are other ways to have -EPERM be returned from
> > > a syscall if you don't have the correct permissions to do something.
> > > Why is seccomp being singled out here?  It's doing the correct thing.
> >
> > AFAIU from what the others have said, it's being singled out because
> > it means that for two semantically equivalent operations (e.g.
> > openat() vs open()), one can fail while the other works because the
> > filter doesn't know about one of the syscalls. Normally semantically
> > equivalent syscalls are supposed to be subject to the same checks, and
> > if one of them fails, trying the other one won't help.
>
> They aren't being subject to the same checks, if the seccomp permissions
> are different for both of them, they will get different answers.
>
> Trying to use this to determine if the syscall is present or not is not
> ok, and as Christian just said, needs to be fixed in userspace.  We
> can't change the kernel ABI now, odds are someone else relies on the api
> we have had in place and it can not be changed :)

I don't think anyone was proposing changes to existing kernel API.