Re: [PATCH RFC 02/24] mm: userfault: return VM_FAULT_RETRY on signals

From: Peter Xu
Date: Tue Jan 22 2019 - 01:11:01 EST

On Mon, Jan 21, 2019 at 10:40:18AM -0500, Jerome Glisse wrote:
> On Mon, Jan 21, 2019 at 03:57:00PM +0800, Peter Xu wrote:
> > There was a special path in handle_userfault() in the past that we'll
> > return a VM_FAULT_NOPAGE when we detected non-fatal signals when waiting
> > for userfault handling. We did that by reacquiring the mmap_sem before
> > returning. However that brings a risk in that the vmas might have
> > changed when we retake the mmap_sem and even we could be holding an
> > invalid vma structure. The problem was reported by syzbot.
> This is confusing this should be a patch on its own ie changes to
> fs/userfaultfd.c where you remove that path.

Sure I will.

> >
> > This patch removes the special path and we'll return a VM_FAULT_RETRY
> > with the common path even if we have got such signals. Then for all the
> > architectures that is passing in VM_FAULT_ALLOW_RETRY into
> > handle_mm_fault(), we check not only for SIGKILL but for all the rest of
> > userspace pending signals right after we returned from
> > handle_mm_fault().
> >
> > The idea comes from the upstream discussion between Linus and Andrea:
> >
> >
> >
> > (This patch contains a potential fix for a double-free of mmap_sem on
> > ARC architecture; please see for
> > more information)
> This patch should only be about changing the return to userspace rule.
> Before this patch the arch fault handler returned to userspace only
> for fatal signal, after this patch it returns to userspace for any
> signal.

Ok. I'll make the first patch to do the signal changes, then the
second patch to remove the userfault path explicitly.

> It would be a lot better to have a fix for arc as a separate patch so
> that we can focus on reviewing only one thing.

I just noticed that it was fixed just a few days ago in commit
4d447455e73b. Then I'll just simply rebase to Linus master and use
the upstream fix, then I can drop this paragraph.

Thanks for the review!

Peter Xu