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

[Jinjiang Tu]

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.

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