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

[Jens Axboe]

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?
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?

-- 
Jens Axboe