linux-kernel - Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

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Date:   Mon, 21 Dec 2020 17:00:22 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Xu <peterx@...hat.com>, Nadav Amit <nadav.amit@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        stable <stable@...r.kernel.org>,
        Minchan Kim <minchan@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

On Mon, Dec 21, 2020 at 03:33:30PM -0800, Linus Torvalds wrote:
> On Mon, Dec 21, 2020 at 3:12 PM Yu Zhao <yuzhao@...gle.com> wrote:
> >
> > I can't say I disagree with you but the man has made the call and I
> > think we should just move on.
> 
> "The man" can always be convinced by numbers.
> 
> So if somebody comes up with an alternate patch, and explains it, and
> shows that it is better - go for it.
> 
> I just think that if mprotect() can take the mmap lock for writing,
> then userfaultfd sure as hell can. What odd load does people have
> where userfaultfd is more important than mprotect?
> 
> So as far as the man is concerned, I think "just fix userfaultfd" is
> simply the default obvious operation.
> 
> Not necessarily a final endpoint.
> 
>                  Linus

My first instinct is to be conservative and revert 09854ba94c6a ("mm:
do_wp_page() simplification") so people are less likely to come back
and complain about performance issues from holding mmap lock for
write when clearing pte_write.

That being said, I do like the simplicity of 09854ba94c6a as well as
having one simple rule that dictates what we should do when clearing
pte_write(). And "userfaultfd is not the most important part of the
system" is a fair point.