[PATCH 0/2] Fixes for SW PAN

[Vinayak Menon]

On 12/6/2017 11:56 PM, Will Deacon wrote:
> On Wed, Dec 06, 2017 at 06:18:01PM +0000, Catalin Marinas wrote:
>> On Wed, Dec 06, 2017 at 06:07:07PM +0000, Will Deacon wrote:
>>> On Wed, Dec 06, 2017 at 06:01:35PM +0000, Catalin Marinas wrote:
>>>> On Wed, Dec 06, 2017 at 05:56:42PM +0000, Will Deacon wrote:
>>>>> On Wed, Dec 06, 2017 at 11:01:46PM +0530, Vinayak Menon wrote:
>>>>>> On 12/6/2017 4:46 PM, Will Deacon wrote:
>>>>>>> After lots of collective head scratching in response to Vinayak's mail
>>>>>>> here:
>>>>>>>
>>>>>>>   http://lists.infradead.org/pipermail/linux-arm-kernel/2017-December/545641.html
>>>>>>>
>>>>>>> It turns out that we have a problem with SW PAN and kernel threads, where
>>>>>>> the saved ttbr0 value for a kernel thread can be stale and subsequently
>>>>>>> inherited by other kernel threads over a fork.
>>>>>>>
>>>>>>> These two patches attempt to fix that. We've not be able to reproduce
>>>>>>> the exact failure reported above, but I added some assertions to the
>>>>>>> uaccess routines to check for discrepancies between the active_mm pgd
>>>>>>> and the saved ttbr0 value (ignoring the zero page) and these no longer
>>>>>>> fire with these changes, but do fire without them if EFI runtime services
>>>>>>> are enabled on my Seattle board.
>>>>>> Thanks Will. So these 2 patches fix the case of kthreads having a stale saved ttbr0. The callstack I had shared
>>>>>> in the original issue description was not of a kthread (its user task with PF_KTHREAD not set. The tsk->mm was
>>>>>> set to NULL by exit_mm I think). So do you think this could be a different problem ?
>>>>>> I had a look at the dumps again and what I see is that, the PA part of the saved ttbr0
>>>>>> (from thread_info) is not the same as the pa(tsk->active_mm->pgd). The PA derived from saved ttbr0 actually
>>>>>> points to a page which is "now" owned by slab.
>>>>> Having not been able to reproduce the failure you described, I can't give
>>>>> you a good answer to this.
> Looking at the code (again), if we context switch in do_exit after exit_mm,
> then the thread behaves an awful lot like a kernel thread: current->mm is
> NULL and we're in lazy TLB mode.
Yes, that could be the case.
I am going to try out these 2 patches and see if the issue gets resolved. It usually takes more
than a day to reproduce the problem. Will update you as soon as I get the results.

> Furthermore, that context switch will drop
> the last reference to the old mm and the pgd will finally be freed.
>
> So I think my patches will solve your case too because we'll call
> enter_lazy_tlb again when getting scheduled back in. If you have any way
> to test them, that would be great.