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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 9 Aug 2022 13:14:49 -0700
Message-ID: <CAHk-=whg0ddey-LqFAPfZJDXHMjaHJNojAV3q17yvjc6W8QRvQ@mail.gmail.com>
Subject: Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW
To:     David Hildenbrand <david@redhat.com>
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>
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On Tue, Aug 9, 2022 at 1:07 PM David Hildenbrand <david@redhat.com> wrote:
>
> >         /* But FOLL_FORCE has no effect on shared mappings */
> >         if (vma->vm_flags & MAP_SHARED)
> >                 return false;
>
> I'd actually rather check for MAP_MAYSHARE here, which is even stronger.
> Thoughts?

Hmm. Adding the test for both is basically free (all those vm_flags
checks end up being a bit mask and compare), so no objections.

For some reason I though VM_SHARED and VM_MAYSHARE end up always being
the same bits, and it was a mistake to make them two bits in the first
place (unlike the read/write/exec bits that can are about mprotect),

But as there are two bits, I'm sure somebody ends up touching one and
not the other.

So yeah, I don't see any downside to just checking both bits.

[ That said, is_cow_mapping() only checks VM_SHARED, so if they are
ever different, that's a potential source of confusion ]

               Linus