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

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Date:   Mon, 21 Dec 2020 16:11:01 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Yu Zhao <yuzhao@...gle.com>
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 4:00 PM Yu Zhao <yuzhao@...gle.com> wrote:
>
> 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.

Well, the thing is, that simplificaiton was actually part of fixing a
real regression wrt GUP.

Reverting that would break a308c71bf1e6 ("mm/gup: Remove enfornced COW
mechanism").

And that one was the (better) fix for commit 17839856fd58 that fixed a
real security issue, but did it with a big hammer that then caused
problems for uffd-wp (and some other loads). There's a bit more
context in the merge message in commit b25d1dc9474e Merge branch
'simplify-do_wp_page'.

So while that commit 09854ba94c6a on its own is "just" a
simplification, it's actually part of a bigger series that fixes
serious problems.

                Linus