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

* Jann Horn:

+seccomp maintainers/reviewers
[thread context is at
https://lore.kernel.org/linux-api/87lfer2c0b.fsf@oldenburg2.str.redhat.com/ (local)
]

On Tue, Nov 24, 2020 at 5:49 PM Christoph Hellwig [off-list ref] wrote:

quoted

On Tue, Nov 24, 2020 at 03:08:05PM +0100, Mark Wielaard wrote:

quoted

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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