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

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Date:   Tue, 24 Nov 2020 18:06:38 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Christoph Hellwig <hch@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>
Cc:     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

+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...