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

[Vincenzo Frascino]

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.

> 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?

-- 
Regards,
Vincenzo