Re: [PATCH] seccomp: passthrough uretprobe systemcall without filtering

Eyal Birger
2025-01-18T20:46:00Z

Hןת

On Sat, Jan 18, 2025 at 12:21 PM Kees Cook <kees@kernel.org> wrote:
>
> On Thu, Jan 16, 2025 at 04:55:39PM -0800, Eyal Birger wrote:
> > Since uretprobe is a "kernel implementation detail" system call which is
> > not used by userspace application code directly, it is impractical and
> > there's very little point in forcing all userspace applications to
> > explicitly allow it in order to avoid crashing tracked processes.
>
> How is this any different from sigreturn, rt_sigreturn, or
> restart_syscall? These are all handled explicitly by userspace filters
> already, and I don't see why uretprobe should be any different. Docker
> has had plenty of experience with fixing their seccomp filters for new
> syscalls. For example, many times already a given libc will suddenly
> start using a new syscall when it sees its available, etc.

I think the difference is that this syscall is not part of the process's
code - it is inserted there by another process tracing it.
So this is different than desiring to deploy a new version of a binary
that uses a new libc or a new syscall. Here the case is that there are
three players - the tracer running out of docker, the tracee running in docker,
and docker itself. All three were running fine in a specific kernel version,
but upgrading the kernel now crashes the traced process.

>
> Basically, this is a Docker issue, not a kernel issue.

As mentione above, for all three given binaries, nothing changed - only the
kernel version.

> Seccomp is behaving correctly. I don't want to start making syscalls invisible
> without an extremely good reason. If _anything_ should be invisible, it
> is restart_syscall (which actually IS invisible under certain
> architectures).

I think this syscall is different in that respect for the reasons described.
I don't know if seccomp is behaving correctly when it blocks a kernel
implementation detail that isn't user created. IMHO the fact that this
implementation detail is implemented as a syscall is unfortunate, and I'm
trying to mitigate the result.

Eyal.
>
> -Kees
>
> --
> Kees Cook