linux-kernel - Re: [PATCH v2 4/4] x86/uaccess: Fix implicit cast of _

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Date:   Sat, 30 Mar 2019 01:57:12 +0530
From:   Mukesh Ojha <mojha@...eaurora.org>
To:     Jann Horn <jannh@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Qiaowei Ren <qiaowei.ren@...el.com>
Subject: Re: [PATCH v2 4/4] x86/uaccess: Fix implicit cast of __user pointer


On 3/29/2019 10:00 PM, Jann Horn wrote:
> The first two arguments of __user_atomic_cmpxchg_inatomic() are:
>
>   - `uval` is a kernel pointer into which the old value should be stored
>   - `ptr` is the user pointer on which the cmpxchg should operate
>
> This means that casting `uval` to `__typeof__(ptr)` is wrong. Since `uval`
> is only used once inside the macro, just get rid of __uval and use `(uval)`
> directly.
>
> Signed-off-by: Jann Horn <jannh@...gle.com>

Looks good to me.
Reviewed-by: Mukesh Ojha <mojha@...eaurora.org>

Cheers,
-Mukesh

> ---
>   arch/x86/include/asm/uaccess.h | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 1954dd5552a2..a21f2a2f17bf 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -585,7 +585,6 @@ extern void __cmpxchg_wrong_size(void)
>   #define __user_atomic_cmpxchg_inatomic(uval, ptr, old, new, size)	\
>   ({									\
>   	int __ret = 0;							\
> -	__typeof__(ptr) __uval = (uval);				\
>   	__typeof__(*(ptr)) __old = (old);				\
>   	__typeof__(*(ptr)) __new = (new);				\
>   	__uaccess_begin_nospec();					\
> @@ -661,7 +660,7 @@ extern void __cmpxchg_wrong_size(void)
>   		__cmpxchg_wrong_size();					\
>   	}								\
>   	__uaccess_end();						\
> -	*__uval = __old;						\
> +	*(uval) = __old;						\
>   	__ret;								\
>   })
>