linux-kernel - Re: [PATCH 1/2] mm/mempolicy: Allow lookup

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Message-ID: <20200408142039.GD66033@xz-x1>
Date:   Wed, 8 Apr 2020 10:20:39 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        syzbot+693dc11fcb53120b5559@...kaller.appspotmail.com
Subject: Re: [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal
 signal

On Wed, Apr 08, 2020 at 12:21:28PM +0200, Michal Hocko wrote:
> On Tue 07-04-20 21:40:09, Peter Xu wrote:
> > 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.
> > 
> > Meanwhile, initialize "page" to NULL to avoid potential risk of
> > exploiting the pointer.
> > 
> > Reported-by: syzbot+693dc11fcb53120b5559@...kaller.appspotmail.com
> > Fixes: 4426e945df58 ("mm/gup: allow VM_FAULT_RETRY for multiple times")
> 
> I am not familiar with thic commit but shouldn't gup return ERESTARTSYS
> on a fatal signal?

Hi, Michal,

I do see quite a few usages on -ERESTARTSYS, but also some others,
majorly -EINTR, or even -EFAULT.  I think it could be a more general
question rather than a specific question to this patch only.

I saw some other discussions about this return value issue, I'll CC
you in the other thread when I raise this as a general question.

Thanks,

-- 
Peter Xu