[PATCH] powerpc/fault: fix wrong KUAP fault for IO_URING

[Jens Axboe]

On 1/27/21 8:13 PM, Zorro Lang wrote:
> On Thu, Jan 28, 2021 at 10:18:07AM +1000, Nicholas Piggin wrote:
>> Excerpts from Jens Axboe's message of January 28, 2021 5:29 am:
>>> On 1/27/21 9:38 AM, Christophe Leroy wrote:
>>>>
>>>>
>>>> Le 27/01/2021 à 15:56, Zorro Lang a écrit :
>>>>> On powerpc, io_uring test hit below KUAP fault on __do_page_fault.
>>>>> The fail source line is:
>>>>>
>>>>>    if (unlikely(!is_user && bad_kernel_fault(regs, error_code, address, is_write)))
>>>>>        return SIGSEGV;
>>>>>
>>>>> The is_user() is based on user_mod(regs) only. This's not suit for
>>>>> io_uring, where the helper thread can assume the user app identity
>>>>> and could perform this fault just fine. So turn to use mm to decide
>>>>> if this is valid or not.
>>>>
>>>> I don't understand why testing is_user would be an issue. KUAP purpose
>>>> it to block any unallowed access from kernel to user memory
>>>> (Equivalent to SMAP on x86). So it really must be based on MSR_PR bit,
>>>> that is what is_user provides.
>>>>
>>>> If the kernel access is legitimate, kernel should have opened
>>>> userspace access then you shouldn't get this "Bug: Read fault blocked
>>>> by KUAP!".
>>>>
>>>> As far as I understand, the fault occurs in
>>>> iov_iter_fault_in_readable() which calls fault_in_pages_readable() And
>>>> fault_in_pages_readable() uses __get_user() so it is a legitimate
>>>> access and you really should get a KUAP fault.
>>>>
>>>> So the problem is somewhere else, I think you proposed patch just
>>>> hides the problem, it doesn't fix it.
>>>
>>> If we do kthread_use_mm(), can we agree that the user access is valid?
>>
>> Yeah the io uring code is fine, provided it uses the uaccess primitives 
>> like any other kernel code. It's looking more like a an arch/powerpc bug.
>>
>>> We should be able to copy to/from user space, and including faults, if
>>> that's been done and the new mm assigned. Because it really should be.
>>> If SMAP was a problem on x86, we would have seen it long ago.
>>>
>>> I'm assuming this may be breakage related to the recent uaccess changes
>>> related to set_fs and friends? Or maybe recent changes on the powerpc
>>> side?
>>>
>>> Zorro, did 5.10 work?
>>
>> Would be interesting to know.
> 
> Sure Nick and Jens, which 5.10 rc? version do you want to know ? Or any git
> commit(be the HEAD) in 5.10 phase?

I forget which versions had what series of this, but 5.10 final - and if
that fails, then 5.9 final. IIRC, 5.9 was pre any of these changes, and
5.10 definitely has them.

-- 
Jens Axboe