linux-kernel - Re: [PATCH RFC 02/24] mm: userfault: return VM_FAULT

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Date:   Tue, 22 Jan 2019 14:10:37 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Shaohua Li <shli@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Pavel Emelyanov <xemul@...allels.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Marty McFadden <mcfadden8@...l.gov>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH RFC 02/24] mm: userfault: return VM_FAULT_RETRY on signals

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:
> > 
> >   https://lkml.org/lkml/2017/10/30/560
> > 
> > (This patch contains a potential fix for a double-free of mmap_sem on
> >  ARC architecture; please see https://lkml.org/lkml/2018/11/1/723 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