linux-kernel - Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory

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Message-ID: <5994adb0-12b7-b8f1-ecdc-ebf56af48947@intel.com>
Date:   Fri, 30 Sep 2022 13:37:54 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
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Subject: Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack
 memory

On 9/30/22 13:30, Edgecombe, Rick P wrote:
> On Fri, 2022-09-30 at 12:16 -0700, Dave Hansen wrote:
>> On 9/29/22 15:29, Rick Edgecombe wrote:
>>> @@ -1633,6 +1633,9 @@ static inline bool
>>> __pte_access_permitted(unsigned long pteval, bool write)
>>>   {
>>>        unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;
>>>
>>> +     if (write && (pteval & (_PAGE_RW | _PAGE_DIRTY)) ==
>>> _PAGE_DIRTY)
>>> +             return 0;
>> Do we not have a helper for this?  Seems a bit messy to open-code
>> these
>> shadow-stack permissions.  Definitely at least needs a comment.
> It's because pteval is an unsigned long. We could create a pte_t, and
> use the helpers, but then we would be using pte_foo() on pmd's, etc. So
> probably comment is the better option?

Yeah, a comment is probably best.

This is one of those "generic" page table functions that doesn't work
well with the p{te,md,ud}_* types.  It's either this or cast over to a
pteval_t for pmd/pud and pretend this is a pte-only function.