linux-kernel - Re: [PATCH] arm64: fix function types in COND

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Date:   Wed, 11 Sep 2019 16:15:46 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: fix function types in COND_SYSCALL

On Tue, Sep 10, 2019 at 03:40:44PM -0700, Sami Tolvanen wrote:
> Define a weak function in COND_SYSCALL instead of a weak alias to
> sys_ni_syscall, which has an incompatible type. This fixes indirect
> call mismatches with Control-Flow Integrity (CFI) checking.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

This looks correct to me, builds fine, and I asume has been tested, so FWIW:

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

In looking at this, I came to the conclusion that we can drop the ifdeffery
around our SYSCALL_DEFINE0(), COND_SYSCALL(), and SYS_NI(), which I evidently
cargo-culted from x86 (where the ifdeffery is actually necessary).

I can send a follow up for that.

Thanks,
Mark.

> ---
>  arch/arm64/include/asm/syscall_wrapper.h | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/syscall_wrapper.h b/arch/arm64/include/asm/syscall_wrapper.h
> index 507d0ee6bc69..06d880b3526c 100644
> --- a/arch/arm64/include/asm/syscall_wrapper.h
> +++ b/arch/arm64/include/asm/syscall_wrapper.h
> @@ -8,6 +8,8 @@
>  #ifndef __ASM_SYSCALL_WRAPPER_H
>  #define __ASM_SYSCALL_WRAPPER_H
>  
> +struct pt_regs;
> +
>  #define SC_ARM64_REGS_TO_ARGS(x, ...)				\
>  	__MAP(x,__SC_ARGS					\
>  	      ,,regs->regs[0],,regs->regs[1],,regs->regs[2]	\
> @@ -35,8 +37,11 @@
>  	ALLOW_ERROR_INJECTION(__arm64_compat_sys_##sname, ERRNO);			\
>  	asmlinkage long __arm64_compat_sys_##sname(const struct pt_regs *__unused)
>  
> -#define COND_SYSCALL_COMPAT(name) \
> -	cond_syscall(__arm64_compat_sys_##name);
> +#define COND_SYSCALL_COMPAT(name) 							\
> +	asmlinkage long __weak __arm64_compat_sys_##name(const struct pt_regs *regs)	\
> +	{										\
> +		return sys_ni_syscall();						\
> +	}
>  
>  #define COMPAT_SYS_NI(name) \
>  	SYSCALL_ALIAS(__arm64_compat_sys_##name, sys_ni_posix_timers);
> @@ -70,7 +75,11 @@
>  #endif
>  
>  #ifndef COND_SYSCALL
> -#define COND_SYSCALL(name) cond_syscall(__arm64_sys_##name)
> +#define COND_SYSCALL(name)							\
> +	asmlinkage long __weak __arm64_sys_##name(const struct pt_regs *regs)	\
> +	{									\
> +		return sys_ni_syscall();					\
> +	}
>  #endif
>  
>  #ifndef SYS_NI
> -- 
> 2.23.0.162.g0b9fbb3734-goog
>