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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 7 Jan 2021 14:42:17 -0800
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Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Yu Zhao <yuzhao@google.com>, Andy Lutomirski <luto@kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Jason Gunthorpe <jgg@ziepe.ca>, Jan Kara <jack@suse.cz>,
        Kirill Tkhai <ktkhai@virtuozzo.com>
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On Thu, Jan 7, 2021 at 2:31 PM Andrea Arcangeli <aarcange@redhat.com> wrote:
>
> Random memory corruption will still silently materialize as result of
> the speculative lookups in the above scenario.

Explain.

Yes, you'll get random memory corruption if you keep doing wrprotect()
without mmap_sem held for writing.

But I thought we agreed earlier that that is a bug. And I thought the
softdirty code already got it for writing.

           Linus