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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 21 Dec 2020 15:22:37 -0800
Message-ID: <CAHk-=wh-bG4thjXUekLtrCg8FRrdWjtT40ibXXLSm_hzQG8eOw@mail.gmail.com>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
To:     Peter Xu <peterx@redhat.com>
Cc:     Nadav Amit <nadav.amit@gmail.com>, Yu Zhao <yuzhao@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        linux-mm <linux-mm@kvack.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        stable <stable@vger.kernel.org>,
        Minchan Kim <minchan@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
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On Mon, Dec 21, 2020 at 2:30 PM Peter Xu <peterx@redhat.com> wrote:
>
> AFAIU mprotect() is the only one who modifies the pte using the mmap write
> lock.  NUMA balancing is also using read mmap lock when changing pte
> protections, while my understanding is mprotect() used write lock only because
> it manipulates the address space itself (aka. vma layout) rather than modifying
> the ptes, so it needs to.

So it's ok to change the pte holding only the PTE lock, if it's a
*one*way* conversion.

That doesn't break the "re-check the PTE contents" model (which
predates _all_ of the rest: NUMA, userfaultfd, everything - it's
pretty much the original model for our page table operations, and goes
back to the dark ages even before SMP and the existence of a page
table lock).

So for example, a COW will always create a different pte (not just
because the page number itself changes - you could imagine a page
getting re-used and changing back - but because it's always a RO->RW
transition).

So two COW operations cannot "undo" each other and fool us into
thinking nothing changed.

Anything that changes RW->RO - like fork(), for example - needs to
take the mmap_lock.

NUMA balancing should be ok wrt COW, because it doesn't do that RW->RO
thing, it uses the present bit.

I think that you are right that NUMA balancing itself might cause
other issues, because it can cause that "pte changed and then came
back" (for numa protectoipn and then a numa fault) all with just the
mmap lock for reading.

However, even that shouldn't matter for COW, because the write protect
bit is the one that proptects the *contents* of the page, so even if
NUMA balancing caused that "load original PTE, then re-check later" to
succeed (despite the PTE actually changing in the middle), the
_contents_ of the page cannot have changed, so COW is ok. NUMA
balancing won't be making a read-only page temporarily writable.

           Linus