Re: [PATCH v14 8/8] kselftest/arm64: Verify that TCO is enabled in load_unaligned_zeropad()

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

On 3/11/21 4:28 PM, Catalin Marinas wrote:
> On Thu, Mar 11, 2021 at 03:00:26PM +0000, Vincenzo Frascino wrote:
>> On 3/11/21 1:25 PM, Catalin Marinas wrote:
>>> On Mon, Mar 08, 2021 at 04:14:34PM +0000, Vincenzo Frascino wrote:
>>>> load_unaligned_zeropad() and __get/put_kernel_nofault() functions can
>>>> read passed some buffer limits which may include some MTE granule with a
>>>> different tag.
>>>>
>>>> When MTE async mode is enable, the load operation crosses the boundaries
>>>> and the next granule has a different tag the PE sets the TFSR_EL1.TF1
>>>> bit as if an asynchronous tag fault is happened:
>>>>
>>>>  ==================================================================
>>>>  BUG: KASAN: invalid-access
>>>>  Asynchronous mode enabled: no access details available
>>>>
>>>>  CPU: 0 PID: 1 Comm: init Not tainted 5.12.0-rc1-ge1045c86620d-dirty #8
>>>>  Hardware name: FVP Base RevC (DT)
>>>>  Call trace:
>>>>    dump_backtrace+0x0/0x1c0
>>>>    show_stack+0x18/0x24
>>>>    dump_stack+0xcc/0x14c
>>>>    kasan_report_async+0x54/0x70
>>>>    mte_check_tfsr_el1+0x48/0x4c
>>>>    exit_to_user_mode+0x18/0x38
>>>>    finish_ret_to_user+0x4/0x15c
>>>>  ==================================================================
>>>>
>>>> Verify that Tag Check Override (TCO) is enabled in these functions before
>>>> the load and disable it afterwards to prevent this to happen.
>>>>
>>>> Note: The issue has been observed only with an MTE enabled userspace.
>>>
>>> The above bug is all about kernel buffers. While userspace can trigger
>>> the relevant code paths, it should not matter whether the user has MTE
>>> enabled or not. Can you please confirm that you can still triggered the
>>> fault with kernel-mode MTE but non-MTE user-space? If not, we may have a
>>> bug somewhere as the two are unrelated: load_unaligned_zeropad() only
>>> acts on kernel buffers and are subject to the kernel MTE tag check fault
>>> mode.
>>
>> I retried and you are right, it does not matter if it is a MTE or non-MTE
>> user-space. The issue seems to be that this test does not trigger the problem
>> all the times which probably lead me to the wrong conclusions.
> 
> Keep the test around for some quick checks before you get the kasan
> test support.
> 

Of course, I never throw away my code.

>>> I don't think we should have a user-space selftest for this. The bug is
>>> not about a user-kernel interface, so an in-kernel test is more
>>> appropriate. Could we instead add this to the kasan tests and calling
>>> load_unaligned_zeropad() and other functions directly?
>>
>> I agree with you we should abandon this strategy of triggering the issue due to
>> my comment above. I will investigate the option of having a kasan test and try
>> to come up with one that calls the relevant functions directly. I would prefer
>> though, since the rest of the series is almost ready, to post it in a future
>> series. What do you think?
> 
> That's fine by me.
> 

-- 
Regards,
Vincenzo