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 2025/3/8 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？

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,
>