linux-kernel - Re: [PATCH 2/3] arm64: syscall.h: Add sign extension handling in syscall_get_return

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Date:   Wed, 21 Apr 2021 18:41:05 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     He Zhe <zhe.he@...driver.com>
Cc:     oleg@...hat.com, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, paul@...l-moore.com,
        eparis@...hat.com, linux-audit@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] arm64: syscall.h: Add sign extension handling in
 syscall_get_return_value for compat

On Fri, Apr 16, 2021 at 03:55:32PM +0800, He Zhe wrote:
> Add sign extension handling in syscall_get_return_value so that it can
> handle 32-bit compatible case and can be used by for example audit, just
> like what syscall_get_error does.

If a compat syscall can ever legitimately return a non-error value with
bit 31 set, and this sign-extends it, is that ever going to reach
userspace as a 64-bit value?

IIUC things like mmap() can return pointers above 2GiB for a compat
task, so I'm a bit uneasy that we'd handle those wrong. I can't see a
way of preventing that unless we keep the upper 32 bits for errors.

Mark.

> 
> Signed-off-by: He Zhe <zhe.he@...driver.com>
> ---
>  arch/arm64/include/asm/syscall.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
> index cfc0672013f6..cd7a22787aeb 100644
> --- a/arch/arm64/include/asm/syscall.h
> +++ b/arch/arm64/include/asm/syscall.h
> @@ -44,7 +44,12 @@ static inline long syscall_get_error(struct task_struct *task,
>  static inline long syscall_get_return_value(struct task_struct *task,
>  					    struct pt_regs *regs)
>  {
> -	return regs->regs[0];
> +	long val = regs->regs[0];
> +
> +	if (is_compat_thread(task_thread_info(task)))
> +		val = sign_extend64(val, 31);
> +
> +	return val;
>  }
>  
>  static inline void syscall_set_return_value(struct task_struct *task,
> -- 
> 2.17.1
>