Re: [PATCH v2 seccomp 4/6] seccomp/cache: Lookup syscall allowlist for fast path

[YiFei Zhu]

On Thu, Sep 24, 2020 at 6:46 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> This protects us from x32 (i.e. syscall_nr will have 0x40000000 bit
> set), but given the effort needed to support compat, I think supporting
> x32 isn't much more. (Though again, I note that NR_syscalls differs in
> size, so this test needs to be per-arch and obviously after
> arch-discovery.)
>
> That said, if it really does turn out that x32 is literally the only
> architecture doing these shenanigans (and I suspect not, given the MIPS
> case), okay, fine, I'll give in. :) You and Jann both seem to think this
> isn't worth it.

MIPS has the sparse syscall shenanigans... idek how that works. Maybe
someone can clarify?

> I think this linear search for the matching arch can be made O(1) (this
> is what I was trying to do in v1: we can map all possible combos to a
> distinct bitmap, so there is just math and lookup rather than a linear
> compare search. In the one-arch case, it can also be easily collapsed
> into a no-op (though my v1 didn't do this correctly).

I remember yours was:

static inline u8 seccomp_get_arch(u32 syscall_arch, u32 syscall_nr)
{
[...]
        switch (syscall_arch) {
        case SECCOMP_ARCH:
                seccomp_arch = SECCOMP_ARCH_IS_NATIVE;
                break;
#ifdef CONFIG_COMPAT
        case SECCOMP_ARCH_COMPAT:
                seccomp_arch = SECCOMP_ARCH_IS_COMPAT;
                break;
#endif
        default:
                seccomp_arch = SECCOMP_ARCH_IS_UNKNOWN;
        }

What I'm relying on here is that the compiler will unroll the loop.
How does the compiler perform switch statements? I was imagining it
would be similar, with "case" corresponding to a compare on the
immediate, and the assign as a move to a register, and break
corresponding to a jump. this would also be O(n) to the number of
arches. Yes, compilers can also do an O(1) table lookup, but that is
nonsensical here -- the arch numbers occupy the MSBs.

That said, does O(1) or O(n) matter here? Given that n is at most 3
you might as well consider it a constant.

Also, does "collapse in one arch case" actually worth it? Given that
there's a likely(), and the other side is a WARN_ON_ONCE(), the
compiler will layout the likely path in the fast path and branch
prediction will be in our favor, right?

YiFei Zhu