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Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for
 shadow stack
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@amacapital.net>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromiun.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
References: <20180710222639.8241-1-yu-cheng.yu@intel.com>
 <20180710222639.8241-17-yu-cheng.yu@intel.com>
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From:   Dave Hansen <dave.hansen@linux.intel.com>
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Message-ID: <5fea4183-d596-5776-d927-552847c52a87@linux.intel.com>
Date:   Tue, 17 Jul 2018 16:11:57 -0700
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On 07/17/2018 04:03 PM, Yu-cheng Yu wrote:
> On Fri, 2018-07-13 at 11:26 -0700, Dave Hansen wrote:
>> On 07/11/2018 10:05 AM, Yu-cheng Yu wrote:
>>>
>>> My understanding is that we don't want to follow write pte if the page
>>> is shared as read-only.  For a SHSTK page, that is (R/O + DIRTY_SW),
>>> which means the SHSTK page has not been COW'ed.  Is that right?
>> Let's look at the code again:
>>
>>>
>>> -static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
>>> +static inline bool can_follow_write_pte(pte_t pte, unsigned int flags,
>>> +					bool shstk)
>>>  {
>>> +	bool pte_cowed = shstk ? is_shstk_pte(pte):pte_dirty(pte);
>>> +
>>>  	return pte_write(pte) ||
>>> -		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
>>> +		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_cowed);
>>>  }
>> This is another case where the naming of pte_*() is biting us vs. the
>> perversion of the PTE bits.  The lack of comments and explanation inthe
>> patch is compounding the confusion.
>>
>> We need to find a way to differentiate "someone can write to this PTE"
>> from "the write bit is set in this PTE".
>>
>> In this particular hunk, we need to make it clear that pte_write() is
>> *never* true for shadowstack PTEs.  In other words, shadow stack VMAs
>> will (should?) never even *see* a pte_write() PTE.
>>
>> I think this is a case where you just need to bite the bullet and
>> bifurcate can_follow_write_pte().  Just separate the shadowstack and
>> non-shadowstack parts.
> 
> In case I don't understand the exact issue.
> What about the following.
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index fc5f98069f4e..45a0837b27f9 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -70,6 +70,12 @@ static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
>  		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
>  }
>  
> +static inline bool can_follow_write_shstk_pte(pte_t pte, unsigned int flags)
> +{
> +	return ((flags & FOLL_FORCE) && (flags & FOLL_COW) &&
> +		is_shstk_pte(pte));
> +}
> +
>  static struct page *follow_page_pte(struct vm_area_struct *vma,
>  		unsigned long address, pmd_t *pmd, unsigned int flags)
>  {
> @@ -105,9 +111,16 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>  	}
>  	if ((flags & FOLL_NUMA) && pte_protnone(pte))
>  		goto no_page;
> -	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
> -		pte_unmap_unlock(ptep, ptl);
> -		return NULL;
> +	if (flags & FOLL_WRITE) {
> +		if (is_shstk_mapping(vma->vm_flags)) {
> +			if (!can_follow_write_shstk_pte(pte, flags)) {
> +				pte_unmap_unlock(ptep, ptl);
> +				return NULL;
> +			}
> +		} else if (!can_follow_write_pte(pte, flags) {
> +			pte_unmap_unlock(ptep, ptl);
> +			return NULL;
> +		}

That looks pretty horrible. :(

We need:

bool can_follow_write(vma, pte_t pte, unsigned int flags)
{
	if (!is_shstk_mapping(vma->vm_flags)) {
		// vanilla case here		
	} else {
		// shadowstack case here
	}
}