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    SubjectRe: [patch 05/16] mm: Allow special mappings with user access cleared
    On Wed, Dec 13, 2017 at 10:08:30AM -0800, Linus Torvalds wrote:
    > On Wed, Dec 13, 2017 at 7:54 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    > >
    > > Which is why get_user_pages() _should_ enforce this.
    > >
    > > What use are protection keys if you can trivially circumvent them?
    >
    > No, we will *not* worry about protection keys in get_user_pages().
    >
    > They are not "security". They are a debug aid and safety against random mis-use.
    >
    > In particular, they are very much *NOT* about "trivially circumvent
    > them". The user could just change their mapping thing, for chrissake!
    >
    > We already allow access to PROT_NONE for gdb and friends, very much on purpose.
    >
    > We're not going to make the VM more complex for something that
    > absolutely nobody cares about, and has zero security issues.

    OK, that might have been my phrasing that was off -- mostly because I
    was looking at it from the VM_NOUSER angle, but currently:

    - gup_pte_range() has pte_access_permitted()

    - follow_page_pte() has pte_access_permitted() for FOLL_WRITE only

    All I'm saying is that that is inconsistent and we should change
    follow_page_pte() to use pte_access_permitted() for FOLL_GET, such that
    __get_user_pages_fast() and __get_user_pages() have matching semantics.

    Now, if VM_NOUSER were to live, the above change would ensure write(2)
    cannot read from such VMAs, where the existing test for FOLL_WRITE
    already disallows read(2) from writing to them.

    But even without VM_NOUSER it makes the VM more consistent than it is
    today.

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