[PATCH 3/8] seccomp: Introduce SECCOMP_PIN_ARCHITECTURE

[Andy Lutomirski]

On Wed, Jun 17, 2020 at 8:25 AM Jann Horn <jannh at google.com> wrote:
>
> On Tue, Jun 16, 2020 at 9:49 AM Kees Cook <keescook at chromium.org> wrote:
> > For systems that provide multiple syscall maps based on architectures
> > (e.g. AUDIT_ARCH_X86_64 and AUDIT_ARCH_I386 via CONFIG_COMPAT), allow
> > a fast way to pin the process to a specific syscall mapping, instead of
> > needing to generate all filters with an architecture check as the first
> > filter action.
>
> This seems reasonable; but can we maybe also add X86-specific handling
> for that X32 mess? AFAIK there are four ways to do syscalls with
> AUDIT_ARCH_X86_64:

You're out of date :)  I fixed the mess.

commit 6365b842aae4490ebfafadfc6bb27a6d3cc54757
Author: Andy Lutomirski <luto at kernel.org>
Date:   Wed Jul 3 13:34:04 2019 -0700

    x86/syscalls: Split the x32 syscalls into their own table



>
> 1. normal x86-64 syscall, X32 bit unset (native case)
> 2. normal x86-64 syscall, X32 bit set (for X32 code calling syscalls
> with no special X32 version)

Returns -ENOSYS now if an x32 version was supposed to be used.

> 3. x32-specific syscall, X32 bit unset (never happens legitimately)

Returns -ENOSYS now.

> 4. x32-specific syscall, X32 bit set (for X32 code calling syscalls
> with special X32 version)
>
> (I got this wrong when I wrote the notes on x32 in the seccomp manpage...)
>
> Can we add a flag for AUDIT_ARCH_X86_64 that says either "I want
> native x64-64" (enforcing case 1) or "I want X32" (enforcing case 2 or
> 4, and in case 2 checking that the syscall has no X32 equivalent)? (Of
> course, if the kernel is built without X32 support, we can leave out
> these extra checks.)

No extra checks needed.  Trying to do a syscall with a wrongly-encoded
x32 nr just generates -ENOSYS now.

Henceforth, all new syscalls will have the same number for native and
x32 and will differ only in the presence of the x32 bit.

--Andy