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

From: Peter Xu <peterx@redhat.com>
To: Jerome Glisse <jglisse@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Hugh Dickins <hughd@google.com>, Maya Gokhale <gokhale2@llnl.gov>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Martin Cracauer <cracauer@cons.org>,
	Denis Plotnikov <dplotnikov@virtuozzo.com>,
	Shaohua Li <shli@fb.com>, Andrea Arcangeli <aarcange@redhat.com>,
	Pavel Emelyanov <xemul@parallels.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Marty McFadden <mcfadden8@llnl.gov>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Mel Gorman <mgorman@suse.de>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>
Subject: Re: [PATCH RFC 02/24] mm: userfault: return VM_FAULT_RETRY on signals
Date: Tue, 22 Jan 2019 14:10:37 +0800	[thread overview]
Message-ID: <20190122061037.GA14907@xz-x1> (raw)
In-Reply-To: <20190121154017.GA3711@redhat.com>

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