Re: Semantics of SECCOMP_MODE_STRICT?

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