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

From: Jann Horn <jannh@google.com>
To: Christoph Hellwig <hch@infradead.org>,
	Kees Cook <keescook@chromium.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Will Drewry <wad@chromium.org>
Cc: Mark Wielaard <mark@klomp.org>,
	Florian Weimer <fweimer@redhat.com>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Linux API <linux-api@vger.kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	dev@opencontainers.org, Jonathan Corbet <corbet@lwn.net>,
	"Carlos O'Donell" <carlos@redhat.com>
Subject: Re: [PATCH] syscalls: Document OCI seccomp filter interactions & workaround
Date: Tue, 24 Nov 2020 18:06:38 +0100	[thread overview]
Message-ID: <CAG48ez2ZHPavVU3_2VnRADFQstOM1s+3GwfWsRaEjAA1jYcHDg@mail.gmail.com> (raw)
In-Reply-To: <20201124164546.GA14094@infradead.org>

+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@infradead.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...