linux-kernel - Re: [PATCH 1/2] arm64: fix syscall_fn

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Date:   Fri, 3 May 2019 11:11:53 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64: fix syscall_fn_t type

On Wed, May 01, 2019 at 01:04:50PM -0700, Sami Tolvanen wrote:
> Use const struct pt_regs * instead of struct pt_regs * as
> the argument type to fix indirect call type mismatches with
> Control-Flow Integrity checking.

It's probably worth noting that in <asm/syscall_wrapper.h> all syscall
wrappers take a const struct pt_regs *, which is where the mismatch
comes from.

> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/arm64/include/asm/syscall.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
> index a179df3674a1a..6206ab9bfcfc5 100644
> --- a/arch/arm64/include/asm/syscall.h
> +++ b/arch/arm64/include/asm/syscall.h
> @@ -20,7 +20,7 @@
>  #include <linux/compat.h>
>  #include <linux/err.h>
>  
> -typedef long (*syscall_fn_t)(struct pt_regs *regs);
> +typedef long (*syscall_fn_t)(const struct pt_regs *regs);

For a second I was worried that we modify the regs to assign the return
value, but I see we do that in the syscall.c wrapper, where the pt_regs
argument isn't const.

We certainly chouldn't need to modify the regs when acquiring the
arguments, and as above this matches <asm/syscall_wrapper.h>, so this
looks sound to me.

FWIW:

Reviewed-by: Mark Rutland <mark.rutland@....com>

Thanks,
Mark.