Re: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall_arches[] array

[Kees Cook]

[resend, argh, I didn't reply-all, sorry for the noise]

On Thu, Sep 24, 2020 at 07:44:17AM -0500, YiFei Zhu wrote:
> From: YiFei Zhu <yifeifz2@xxxxxxxxxxxx>
> 
> Seccomp cache emulator needs to know all the architecture numbers
> that syscall_get_arch() could return for the kernel build in order
> to generate a cache for all of them.
> 
> The array is declared in header as static __maybe_unused const
> to maximize compiler optimiation opportunities such as loop
> unrolling.

Disregarding the "how" of this, yeah, we'll certainly need something to
tell seccomp about the arrangement of syscall tables and how to find
them.

However, I'd still prefer to do this on a per-arch basis, and include
more detail, as I've got in my v1.

Something missing from both styles, though, is a consolidation of
values, where the AUDIT_ARCH* isn't reused in both the seccomp info and
the syscall_get_arch() return. The problems here were two-fold:

1) putting this in syscall.h meant you do not have full NR_syscall*
   visibility on some architectures (e.g. arm64 plays weird games with
   header include order).

2) seccomp needs to handle "multiplexed" tables like x86_x32 (distros
   haven't removed CONFIG_X86_X32 widely yet, so it is a reality that
   it must be dealt with), which means seccomp's idea of the arch
   "number" can't be the same as the AUDIT_ARCH.

So, likely a combo of approaches is needed: an array (or more likely,
enum), declared in the per-arch seccomp.h file. And I don't see a way
to solve #1 cleanly.

Regardless, it needs to be split per architecture so that regressions
can be bisected/reverted/isolated cleanly. And if we can't actually test
it at runtime (or find someone who can) it's not a good idea to make the
change. :)

> [...]
> diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
> index 7cbf733d11af..e13bb2a65b6f 100644
> --- a/arch/x86/include/asm/syscall.h
> +++ b/arch/x86/include/asm/syscall.h
> @@ -97,6 +97,10 @@ static inline void syscall_set_arguments(struct task_struct *task,
>  	memcpy(&regs->bx + i, args, n * sizeof(args[0]));
>  }
>  
> +static __maybe_unused const int syscall_arches[] = {
> +	AUDIT_ARCH_I386
> +};
> +
>  static inline int syscall_get_arch(struct task_struct *task)
>  {
>  	return AUDIT_ARCH_I386;
> @@ -152,6 +156,13 @@ static inline void syscall_set_arguments(struct task_struct *task,
>  	}
>  }
>  
> +static __maybe_unused const int syscall_arches[] = {
> +	AUDIT_ARCH_X86_64,
> +#ifdef CONFIG_IA32_EMULATION
> +	AUDIT_ARCH_I386,
> +#endif
> +};

I'm leaving this section quoted because I'll refer to it in a later
patch review...

-- 
Kees Cook