linux-kernel - Re: BUG: unable to handle kernel paging request in kernel_get

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Date:   Mon, 6 Apr 2020 19:15:34 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     syzbot <syzbot+693dc11fcb53120b5559@...kaller.appspotmail.com>,
        bgeffon@...gle.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com,
        torvalds@...ux-foundation.org
Subject: Re: BUG: unable to handle kernel paging request in
 kernel_get_mempolicy

On Mon, 6 Apr 2020 21:55:35 -0400 Peter Xu <peterx@...hat.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@...hat.com> wrote:
> > 
> > > >From 23800bff6fa346a4e9b3806dc0cfeb74498df757 Mon Sep 17 00:00:00 2001
> > > From: Peter Xu <peterx@...hat.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?