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Date:   Fri, 30 Sep 2022 13:37:54 -0700
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Subject: Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack
 memory
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To:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        "bsingharora@gmail.com" <bsingharora@gmail.com>,
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        "Syromiatnikov, Eugene" <esyr@redhat.com>,
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        "keescook@chromium.org" <keescook@chromium.org>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>,
        "Eranian, Stephane" <eranian@google.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
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        "hjl.tools@gmail.com" <hjl.tools@gmail.com>,
        "Yang, Weijiang" <weijiang.yang@intel.com>,
        "Lutomirski, Andy" <luto@kernel.org>,
        "pavel@ucw.cz" <pavel@ucw.cz>, "arnd@arndb.de" <arnd@arndb.de>,
        "Moreira, Joao" <joao.moreira@intel.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mike.kravetz@oracle.com" <mike.kravetz@oracle.com>,
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References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-23-rick.p.edgecombe@intel.com>
 <9fed0342-2d02-aaf2-ed66-20ff08bdfd0b@intel.com>
 <44314145f644ab822ac36cc8c78520d5f34d5bd8.camel@intel.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <44314145f644ab822ac36cc8c78520d5f34d5bd8.camel@intel.com>
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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.