Re: [PATCH v1 0/6] seccomp: Implement constant action bitmaps

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On 24/09/2020 01.29, Kees Cook wrote:
> rfc: https://lore.kernel.org/lkml/20200616074934.1600036-1-keescook@chromium.org/
> alternative: https://lore.kernel.org/containers/cover.1600661418.git.yifeifz2@illinois.edu/
> v1:
> - rebase to for-next/seccomp
> - finish X86_X32 support for both pinning and bitmaps
> - replace TLB magic with Jann's emulator
> - add JSET insn
> 
> TODO:
> - add ALU|AND insn
> - significantly more testing
> 
> Hi,
> 
> This is a refresh of my earlier constant action bitmap series. It looks
> like the RFC was missed on the container list, so I've CCed it now. :)
> I'd like to work from this series, as it handles the multi-architecture
> stuff.

So, I agree with Jann's point that the only thing that matters is that
always-allowed syscalls are indeed allowed fast.

But one thing I'm wondering about and I haven't seen addressed anywhere:
Why build the bitmap on the kernel side (with all the complexity of
having to emulate the filter for all syscalls)? Why can't userspace just
hand the kernel "here's a new filter: the syscalls in this bitmap are
always allowed noquestionsasked, for the rest, run this bpf". Sure, that
might require a new syscall or extending seccomp(2) somewhat, but isn't
that a _lot_ simpler? It would probably also mean that the bpf we do get
handed is a lot smaller. Userspace might need to pass a couple of
bitmaps, one for each relevant arch, but you get the overall idea.

I'm also a bit worried about the performance of doing that emulation;
that's constant extra overhead for, say, launching a docker container.

Regardless of how the kernel's bitmap gets created, something like

+	if (nr < NR_syscalls) {
+		if (test_bit(nr, bitmaps->allow)) {
+			*filter_ret = SECCOMP_RET_ALLOW;
+			return true;
+		}

probably wants some nospec protection somewhere to avoid the irony of
seccomp() being used actively by bad guys.

Rasmus