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Date:   Tue, 9 Aug 2022 22:42:06 +0200
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Subject: Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove
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To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        stable@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Peter Xu <peterx@redhat.com>, Hugh Dickins <hughd@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>
References: <20220808073232.8808-1-david@redhat.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 09.08.22 22:30, Linus Torvalds wrote:
> On Tue, Aug 9, 2022 at 1:20 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> IIUC VM_MAYSHARE is always set in a MAP_SHARED mapping, but for file
>> mappings we only set VM_SHARED if the file allows for writes
> 
> Heh.
> 
> This is a horrific hack, and probably should go away.
> 
> Yeah, we have that
> 
>                         if (!(file->f_mode & FMODE_WRITE))
>                                 vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
> 
> 
> but I think that's _entirely_ historical.
> 
> Long long ago, in a galaxy far away, we didn't handle shared mmap()
> very well. In fact, we used to not handle it at all.
> 
> But nntpd would use write() to update the spool file, adn them read it
> through a shared mmap.
> 
> And since our mmap() *was* coherent with people doing write() system
> calls, but didn't handle actual dirty shared mmap, what Linux used to
> do was to just say "Oh, you want a read-only shared file mmap? I can
> do that - I'll just downgrade it to a read-only _private_ mapping, and
> it actually ends up with the same semantics".
> 
> And here we are, 30 years later, and it still does that, but it leaves
> the VM_MAYSHARE flag so that /proc/<pid>/maps can show that it's a
> shared mapping.

I was suspecting that this code is full of legacy :)

What would make sense to me is to just have VM_SHARED and make it
correspond to MAP_SHARED, that would at least confuse me less. Once I
have some spare cycles I'll see how easy that might be to achieve.

-- 
Thanks,

David / dhildenb