Re: [PATCH] arm64: audit: fix return value high 32bit truncation problem

[Will Deacon]

On Thu, Jul 22, 2021 at 02:07:07PM +0800, Yuchen Wei wrote:
> From: weiyuchen <weiyuchen3@xxxxxxxxxx>
> 
> Add error code judgment in invoke_syscall() to prevent kernel
> components such as audit and tracepoint from obtaining incorrect
> return values. For example:
> 
> type=SYSCALL msg=audit(342.780:69): arch=40000028 syscall=235
> success=yes exit=4294967235
> 
> The syscall return value is -61, but due to the following process in
> invoke_syscall():
> 
> 	if (is_compat_task())
> 		ret = lower_32_bits(ret);
> 	regs->regs[0] = ret;
> 
> The return value audit or tracepoint get from regs[0] is 4294967235,
> which is an incorrect return value.
> 
> Signed-off-by: weiyuchen <weiyuchen3@xxxxxxxxxx>
> ---
>  arch/arm64/kernel/syscall.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
> index 263d6c1a525f..f9f042d9a088 100644
> --- a/arch/arm64/kernel/syscall.c
> +++ b/arch/arm64/kernel/syscall.c
> @@ -54,7 +54,7 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
>  		ret = do_ni_syscall(regs, scno);
>  	}
>  
> -	if (is_compat_task())
> +	if (is_compat_task() && !IS_ERR_VALUE(ret))
>  		ret = lower_32_bits(ret);

Hmm, I'm worried this might break other users who don't expect to see
non-zero bits for the upper 32-bits of a compat task.

Mark -- I remember you looking into this relatively recently. Where did you
get to with it?

Will