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

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Message-ID: <20200407015535.GC48345@xz-x1>
Date:   Mon, 6 Apr 2020 21:55:35 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
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, 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.

Thanks,

-- 
Peter Xu