Re: BUG: unable to handle kernel paging request in kernel_get_mempolicy

Andrew Morton
2020-04-07T02:15:37Z

On Mon, 6 Apr 2020 21:55:35 -0400 Peter Xu <peterx@redhat.com> wrote:

> On Mon, Apr 06, 2020 at 06:39:41PM -0700, Andrew Morton wrote:
> > On Mon, 6 Apr 2020 20:47:45 -0400 Peter Xu <peterx@redhat.com> wrote:
> >
> > > >From 23800bff6fa346a4e9b3806dc0cfeb74498df757 Mon Sep 17 00:00:00 2001
> > > From: Peter Xu <peterx@redhat.com>
> > > Date: Mon, 6 Apr 2020 20:40:13 -0400
> > > Subject: [PATCH] mm/mempolicy: Allow lookup_node() to handle fatal signal
> > >
> > > lookup_node() uses gup to pin the page and get node information. It
> > > checks against ret>=0 assuming the page will be filled in. However
> > > it's also possible that gup will return zero, for example, when the
> > > thread is quickly killed with a fatal signal. Teach lookup_node() to
> > > gracefully return an error -EFAULT if it happens.
> > >
> > > ...
> > >
> > > --- a/mm/mempolicy.c
> > > +++ b/mm/mempolicy.c
> > > @@ -902,7 +902,10 @@ static int lookup_node(struct mm_struct *mm, unsigned long addr)
> > >
> > > int locked = 1;
> > > err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked);
> > > - if (err >= 0) {
> > > + if (err == 0) {
> > > + /* E.g. GUP interupted by fatal signal */
> > > + err = -EFAULT;
> > > + } else if (err > 0) {
> > > err = page_to_nid(p);
> > > put_page(p);
> > > }
> >
> > Doh. Thanks.
> >
> > Should it have been -EINTR?
>
> It looks ok to me too. I was returning -EFAULT to follow the same
> value as get_vaddr_frames() (which is the other caller of
> get_user_pages_locked()). So far the only path that I found can
> trigger this is when there's a fatal signal pending right after the
> gup. If so, the userspace won't have a chance to see the -EINTR (or
> whatever we return) anyways.

Yup. I guess we're a victim of get_user_pages()'s screwy return value
conventions - the caller cannot distinguish between invalid-addr and
fatal-signal.

Which makes one wonder why lookup_node() ever worked. What happens if
get_mempolicy(MPOL_F_NODE) is passed a wild userspace address?