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

[Christophe Leroy]

Le 28/01/2021 à 15:42, Jens Axboe a écrit :
> On 1/28/21 6:52 AM, Zorro Lang wrote:
>> On Wed, Jan 27, 2021 at 08:06:37PM -0700, Jens Axboe wrote:
>>> 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.
>>
>> I justed built linux v5.10 with same .config file, and gave it same test.
>> v5.10 (HEAD=2c85ebc57b Linux 5.10) can't reproduce this bug:
>>
>> # ./check generic/013 generic/051
>> FSTYP         -- xfs (non-debug)
>> PLATFORM      -- Linux/ppc64le ibm-p9z-xxx-xxxx 5.10.0 #3 SMP Thu Jan 28 04:12:14 EST 2021
>> MKFS_OPTIONS  -- -f -m crc=1,finobt=1,reflink=1,rmapbt=1,bigtime=1,inobtcount=1 /dev/sda3
>> MOUNT_OPTIONS -- -o context=system_u:object_r:root_t:s0 /dev/sda3 /mnt/xfstests/scratch
>>
>> generic/013 138s ...  77s
>> generic/051 103s ...  143s
>> Ran: generic/013 generic/051
>> Passed all 2 tests
> 
> Thanks for testing that, so I think it's safe to conclude that there's a
> regression in powerpc fault handling for kthreads that use
> kthread_use_mm in this release. A bisect would definitely find it, but
> might be pointless if Christophe or Nick already have an idea of what it
> is.
> 

I don't have any idea yet, but I'd be curious to see the vmlinux binary matching the reported Oops.

Christophe