Re: [PATCH] MIPS: Fix strnlen_user access check

[Jinyang He]

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@xxxxxxxxxxx>
> ---
>   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;
Sorry for here and it should be ~__UA_LIMIT...
earlier discussed:
https://patchwork.kernel.org/project/linux-mips/patch/1618139092-4018-1-git-send-email-hejinyang@xxxxxxxxxxx/#24107107

Hope other comment. :-)

Jinyang

> +	}
>   
>   	might_fault();
>   	__asm__ __volatile__(