Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <9E301C7C-882A-4E0F-8D6D-1170E792065A@gmail.com>
 <CAHk-=wg-Y+svNy3CDkJjj0X_CJkSbpERLg64-Vqwq5u7SC4z0g@mail.gmail.com>
 <X+ESkna2z3WjjniN@google.com> <1FCC8F93-FF29-44D3-A73A-DF943D056680@gmail.com>
 <20201221223041.GL6640@xz-x1> <CAHk-=wh-bG4thjXUekLtrCg8FRrdWjtT40ibXXLSm_hzQG8eOw@mail.gmail.com>
 <CALCETrV=8tY7h=aaudWBEn-MJnNkm2wz5qjH49SYqwkjYTpOaA@mail.gmail.com>
 <X+JJqK91plkBVisG@redhat.com> <X+JhwVX3s5mU9ZNx@google.com>
 <X+Js/dFbC5P7C3oO@redhat.com> <X+KDwu1PRQ93E2LK@google.com> <CAHk-=wiBWkgxLtwD7n01irD7hTQzuumtrqCkxxZx=6dbiGKUqQ@mail.gmail.com>
In-Reply-To: <CAHk-=wiBWkgxLtwD7n01irD7hTQzuumtrqCkxxZx=6dbiGKUqQ@mail.gmail.com>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 22 Dec 2020 16:01:45 -0800
Message-ID: <CAHk-=wjG7xx7Gsb=K0DteB1SPcKjus02zY2gFUoxMY5mm7tfsA@mail.gmail.com>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
To:     Yu Zhao <yuzhao@google.com>
Cc:     Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Nadav Amit <nadav.amit@gmail.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>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Dec 22, 2020 at 3:50 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> 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).

The more I look at the mprotect code, the less I like it. We seem to
be much better about the TLB flushes in other places (looking at
mremap, for example). The mprotect code seems to be very laissez-faire
about the TLB flushing.

Does adding a TLB flush to before that

        pte_unmap_unlock(pte - 1, ptl);

fix things for you?

That's not the right fix - leaving a stale TLB entry around is fine if
the TLB entry is more strict wrt protections - but it might be worth
testing as a "does it at least close the problem" patch.

          Linus