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Subject: Re: [PATCH] MIPS: Fix strnlen_user access check
To:     Jinyang He <hejinyang@loongson.cn>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>
References: <1618139092-4018-1-git-send-email-hejinyang@loongson.cn>
Cc:     linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org
From:   Tiezhu Yang <yangtiezhu@loongson.cn>
Message-ID: <cbe5e79b-ee6c-5c59-0051-28e4d1152666@loongson.cn>
Date:   Mon, 12 Apr 2021 11:02:19 +0800
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On 04/11/2021 07:04 PM, Jinyang He wrote:
> Commit 04324f44cb69 ("MIPS: Remove get_fs/set_fs") brought a problem for
> strnlen_user(). Jump out when checking access_ok() with condition that
> (s + strlen(s)) < __UA_LIMIT <= (s + n). The old __strnlen_user_asm()
> just checked (ua_limit & s) without checking (ua_limit & (s + n)).
> Therefore, find strlen form s to __UA_LIMIT - 1 in that condition.
>
> Signed-off-by: Jinyang He <hejinyang@loongson.cn>
> ---
>   arch/mips/include/asm/uaccess.h | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/arch/mips/include/asm/uaccess.h b/arch/mips/include/asm/uaccess.h
> index 91bc7fb..85ba0c8 100644
> --- a/arch/mips/include/asm/uaccess.h
> +++ b/arch/mips/include/asm/uaccess.h
> @@ -630,8 +630,15 @@ static inline long strnlen_user(const char __user *s, long n)
>   {
>   	long res;
>   
> -	if (!access_ok(s, n))
> -		return -0;
> +	if (unlikely(n <= 0))
> +		return 0;
> +
> +	if (!access_ok(s, n)) {
> +		if (!access_ok(s, 0))
> +			return 0;
> +
> +		n = __UA_LIMIT - (unsigned long)s - 1;
> +	}
>   
>   	might_fault();
>   	__asm__ __volatile__(

The following simple changes are OK to fix this issue?

diff --git a/arch/mips/include/asm/uaccess.h b/arch/mips/include/asm/uaccess.h
index 91bc7fb..eafc99b 100644
--- a/arch/mips/include/asm/uaccess.h
+++ b/arch/mips/include/asm/uaccess.h
@@ -630,8 +630,8 @@ static inline long strnlen_user(const char __user *s, long n)
  {
         long res;
  
-       if (!access_ok(s, n))
-               return -0;
+       if (!access_ok(s, 1))
+               return 0;
  
         might_fault();
         __asm__ __volatile__(

Thanks,
Tiezhu