Re: Using userfaultfd with KVM's async page fault handling causes processes to hung waiting for mmap_lock to be released

[Jinjiang Tu <tujinjiang@huawei.com>]

在 2025/3/11 16:14, Jinjiang Tu 写道:
>
> 在 2025/3/11 2:50, Peter Xu 写道:
>> On Mon, Mar 10, 2025 at 02:40:35PM +0800, Jinjiang Tu wrote:
>>> 在 2025/3/8 6:41, Peter Xu 写道:
>>>> On Fri, Mar 07, 2025 at 03:11:09PM +0200, jimsiak wrote:
>>>>> Hi,
>>>>>
>>>>>   From my side, I managed to avoid the freezing of processes with the
>>>>> following change in function userfaultfd_release() in file 
>>>>> fs/userfaultfd.c
>>>>> (https://elixir.bootlin.com/linux/v5.13/source/fs/userfaultfd.c#L842): 
>>>>>
>>>>>
>>>>> I moved the following command from line 851:
>>>>> WRITE_ONCE(ctx->released, true);
>>>>> (https://elixir.bootlin.com/linux/v5.13/source/fs/userfaultfd.c#L851)
>>>>>
>>>>> to line 905, that is exactly before the functions returns 0.
>>>>>
>>>>> That simple workaround worked for my use case but I am far from 
>>>>> sure that is
>>>>> a correct/sufficient fix for the problem at hand.
>>>> Updating the field after userfaultfd_ctx_put() might mean UAF, afaict.
>>>>
>>>> Maybe it's possible to remove ctx->released but only rely on the 
>>>> mmap write
>>>> lock.  However that'll need some closer look and more thoughts.
>>>>
>>>> To me, the more straightforward way to fix it is to use the patch I
>>>> mentioned in the other email:
>>>>
>>>> https://lore.kernel.org/all/ZLmT3BfcmltfFvbq@x1n/
>>>>
>>>> Or does it mean it didn't work at all?
>>> This patch works for me. mlock() syscall calls GUP with 
>>> FOLL_UNLOCKABLE and
>>> allows to release mmap lock and retry.
>>>
>>> But other GUP call without FOLL_UNLOCKABLE will return VM_FAULT_SIGBUS,
>>> is it a regression for the below commit？
>> Do you have an explicit reproducer / use case of such?
>>
>> AFAIU, below commit should only change it from SIGBUS to NOPAGE when
>> "released" is set.  I don't see how it can regress on !FOLL_UNLOCKABLE.
>>
>> Thanks,
>
> You are right, the below commit seems to only care about page fault 
> from userspace (which has
> FAULT_FLAG_ALLOW_RETRY flag), and doesn't care about GUP from drivers 
> (which may be !FOLL_UNLOCKABLE)
>
> Thanks.

Hi Peter,

Since this patch works, could you please send a formal patch to maillist?

Thanks.

>
>>> commit 656710a60e3693911bee3a355d2f2bbae3faba33
>>> Author: Andrea Arcangeli <aarcange@redhat.com>
>>> Date:   Fri Sep 8 16:12:42 2017 -0700
>>>
>>>      userfaultfd: non-cooperative: closing the uffd without 
>>> triggering SIGBUS
>>>
>>>> Thanks,
>>>>