Re: [Ksummit-discuss] [TECH TOPIC] seccomp

[Kees Cook <keescook@chromium.org>]

On Fri, Jul 19, 2019 at 05:32:59AM -0700, Andy Lutomirski wrote:
> On Fri, Jul 19, 2019 at 2:35 AM Christian Brauner <christian@brauner.io> wrote:
> >
> > In light of all this, I would argue that we should seriously look into
> > extending seccomp to allow filtering on pointer arguments.

I would be all for this. :) I've struggled for a long while trying to
find a sane design for this.

> I won't be at LPC this year, but I was thinking about this anyway.  I
> have the following suggestion that might be a bit unorthodox: have
> syscalls opt into this filtering.  Specifically, a syscall that
> supports pointer filtering would be refactored the way a bunch of our
> syscalls are already refactored.  The baseline situation is:
> 
> SYSCALL_DEFINE1(syscallname, struct foo __user *, buf) { ... }
> 
> Instead, we would do:
> 
> SYSCALL_FILTERABLE(syscallname, struct foo __user *, buf)
> {
>   int ret;
>   struct foo kbuf;
>   ret = copy_from_user(&kbuf, buf, sizeof(buf));
>   if (ret)
>     return ret;
> 
>   ret = seccomp_deep_filter(syscallname, 0, &kbuf);
>   if (ret)
>     return ret;
> 
>   return do_syscallname(&kbuf);
> }
> 
> In principle, if we know we're doing a FILTERABLE syscall, we could
> skip the initial seccomp invocation and just defer it until
> seccomp_deep_filter(), although this might interact badly with any
> SECCOMP_RET_PTRACE handles that change nr.

I don't like splitting the logic on seccomp invocation (we end up needing
to solve ordering issues maybe again), but I do like this explicit
opt-in feature. How you have it does make the "where do we store a cached
copy?" problem go away, too.

With a solution looming, now my mind turns to "how do we write filters
that check argument data?" Can this be done sanely with cBPF or are we
finally to requiring eBPF?

The placement of the seccomp hook looks rather like an LSM, which gets
me back to earlier LSM hooking designs I'd considered:
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=seccomp/lsm&id=10c1e4d2b51ad61ad516fa44c2e007f3f5f6edfb
Which also didn't solve the split-location of seccomp rules and wasn't
creating a dynamic way to do, say, string matching.

> To make this robust, it might help a lot if the generation of these
> stubs was mostly automated.

Agreed.

-- 
Kees Cook