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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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