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

[Florian Weimer]

* Jann Horn:

> +seccomp maintainers/reviewers
> [thread context is at
> https://lore.kernel.org/linux-api/87lfer2c0b.fsf@xxxxxxxxxxxxxxxxxxxxxxxxx/
> ]
>
> On Tue, Nov 24, 2020 at 5:49 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> 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...

But that's playing Core Wars, right?  Someone will write a seccomp
filter trying to game that kernel check.  I don't really think it solves
anything until there is consensus what a system call filter should do
with system calls not on the permitted list.

Thanks,
Florian
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