Re: [PATCH] MIPS: Fix strnlen_user access check

[Jinyang He <hejinyang@loongson.cn>]

On 04/12/2021 10:27 PM, Thomas Bogendoerfer wrote:

> On Mon, Apr 12, 2021 at 11:02:19AM +0800, Tiezhu Yang wrote:
>> 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__(
> that's the fix I'd like to apply. Could someone send it as a formal
> patch ? Thanks.
>
> Thomas.
>
Hi, Thomas,

Thank you for bringing me more thinking.

I always think it is better to use access_ok(s, 0) on MIPS. I have been
curious about the difference between access_ok(s, 0) and access_ok(s, 1)
until I saw __access_ok() on RISCV at arch/riscv/include/asm/uaccess.h

The __access_ok() is noted with `Ensure that the range [addr, addr+size)
is within the process's address space`. Does the range checked by
__access_ok() on MIPS is [addr, addr+size]. So if we want to use
access_ok(s, 1), should we modify __access_ok()? Or my misunderstanding?

More importantly, the implementation of strnlen_user in lib/strnlen_user.c
is noted `we hit the address space limit, and we still had more characters
the caller would have wanted. That's 0.` Does it make sense? It is not
achieved on MIPS when hit __ua_limit, if only access_ok(s, 1) is used.

Thanks,
Jinyang