linux-kernel - Re: [RFC PATCH v9 14/27] mm: Handle Shadow Stack page fault

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Date:   Tue, 7 Apr 2020 15:21:59 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>, x86-patch-review@...el.com
Subject: Re: [RFC PATCH v9 14/27] mm: Handle Shadow Stack page fault

On 4/7/20 11:14 AM, Yu-cheng Yu wrote:
> On Wed, 2020-02-26 at 16:08 -0800, Dave Hansen wrote:
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index 45442d9a4f52..6daa28614327 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -772,7 +772,8 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>>>  	 * If it's a COW mapping, write protect it both
>>>  	 * in the parent and the child
>>>  	 */
>>> -	if (is_cow_mapping(vm_flags) && pte_write(pte)) {
>>> +	if ((is_cow_mapping(vm_flags) && pte_write(pte)) ||
>>> +	    arch_copy_pte_mapping(vm_flags)) {
>>>  		ptep_set_wrprotect(src_mm, addr, src_pte);
>>>  		pte = pte_wrprotect(pte);
>>>  	}
>>
>> You have to modify this because pte_write()==0 for shadow stack PTEs, right?
>>
>> Aren't shadow stack ptes *logically* writable, even if they don't have
>> the write bit set?  What would happen if we made pte_write()==1 for them?
> 
> Here the vm_flags needs to have VM_MAYWRITE, and the PTE needs to have
> _PAGE_WRITE.  A shadow stack does not have either.

I literally mean taking pte_write(), and doing something l

static inline int pte_write(pte_t pte)
{
	if (pte_present(pte) && pte_is_shadow_stack(pte))
		return 1;

        return pte_flags(pte) & _PAGE_RW;
}

Then if is_cow_mapping() returns true for shadow stack VMAs, the above
code doesn't need to change.

> To fix checking vm_flags, what about adding a "arch_is_cow_mappping()" to the
> generic is_cow_mapping()?

That makes good sense to me.

> For the PTE, the check actually tries to determine if the PTE is not already
> being copy-on-write, which is:
> 
> 	(!_PAGE_RW && !_PAGE_DIRTY_HW)
> 
> So what about making it pte_cow()?
> 
> 	/*
> 	 * The PTE is in copy-on-write status.
> 	 */
> 	static inline int pte_cow(pte_t pte)
> 	{
> 		return !(pte_flags(pte) & (_PAGE_WRITE | _PAGE_DIRTY_HW));
> 	}

... with appropriate comments that seems fine to me.