Re: Semantics of SECCOMP_MODE_STRICT?

Kees Cook
2021-06-30T05:23:11Z

On Tue, Jun 29, 2021 at 05:54:24PM -0500, Eric W. Biederman wrote:
>
> I am the process of cleaning up the process exit path in the kernel, and
> as part of that I am looking at the callers of do_exit. A very
> interesting one is __seccure_computing_strict.
>
> Looking at the code is very clear that if a system call is attempted
> that is not in the table the thread attempting to execute that system
> call is terminated.
>
> Reading the man page for seccomp it says that the process is delivered
> SIGKILL.
>
> The practical difference is what happens for multi-threaded
> applications.
>
> What are the desired semantics for a multi-threaded application if one
> thread attempts to use a unsupported system call? Should the thread be
> terminated or the entire application?
>
> Do we need to fix the kernel, or do we need to fix the manpages?

I don't know of anyone actually using SECCOMP_MODE_STRICT, but the
original implementation was (perhaps accidentally) thread-killing. It
turns out this is not a particularly desirable situation, and when
SECCOMP_MODE_FILTER was created, it continued with that semantic,
but later grew a process-killing flags, as that's what most programs
actually wanted.

It's likely the manpage needs fixing (we had to make similar updates
for SECCOMP_MODE_FILTER), since some of the early examples of using
SECCOMP_MODE_STRICT were basically "fork, calculate, write result to
fd, exit".

FWIW the seccomp selftests don't even check for the thread-vs-process
SIGKILL of SECCOMP_MODE_STRICT. :)

--
Kees Cook