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    SubjectRe: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW
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    > On Aug 30, 2018, at 10:59 AM, Jann Horn <jannh@google.com> wrote:
    >
    >> On Thu, Aug 30, 2018 at 7:58 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
    >>
    >>> On Thu, 2018-08-30 at 10:33 -0700, Dave Hansen wrote:
    >>>> On 08/30/2018 10:26 AM, Yu-cheng Yu wrote:
    >>>>
    >>>> We don't have the guard page now, but there is a shadow stack
    >>>> token
    >>>> there, which cannot be used as a return address.
    >>> The overall concern is that we could overflow into a page that we
    >>> did
    >>> not intend. Either another actual shadow stack or something that a
    >>> page
    >>> that the attacker constructed, like the transient scenario Jann
    >>> described.
    >>>
    >>
    >> A task could go beyond the bottom of its shadow stack by doing either
    >> 'ret' or 'incssp'. If it is the 'ret' case, the token prevents it.
    >> If it is the 'incssp' case, a guard page cannot prevent it entirely,
    >> right?
    >
    > I mean the other direction, on "call".

    I still think that shadow stacks should work just like mmap and that mmap should learn to add guard pages for all non-MAP_FIXED allocations.
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