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

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Message-ID: <CAHk-=wiBWkgxLtwD7n01irD7hTQzuumtrqCkxxZx=6dbiGKUqQ@mail.gmail.com>
Date:   Tue, 22 Dec 2020 15:50:29 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Nadav Amit <nadav.amit@...il.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>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

On Tue, Dec 22, 2020 at 3:39 PM Yu Zhao <yuzhao@...gle.com> wrote:
>
> 2) is the false positive because of what we do, and it's causing the
> memory corruption because do_wp_page() tries to make copies of pages
> that seem to be RO but may have stale RW tlb entries pending flush.

Yeah, that's definitely a different bug.

The rule is that the TLB flush has to be done before the page table
lock is released.

See zap_pte_range() for an example of doing it right, even in the
presence of complexities (ie that has an example of both flushing the
TLB, and doing the actual "free the pages after flush", and it does
the two cases separately).

           Linus