Re: [PATCH v20 08/25] x86/mm: Introduce _PAGE_COW

["Yu, Yu-cheng" <yu-cheng.yu@intel.com>]

On 2/10/2021 11:42 AM, Kees Cook wrote:
> On Wed, Feb 10, 2021 at 09:56:46AM -0800, Yu-cheng Yu wrote:
>> There is essentially no room left in the x86 hardware PTEs on some OSes
>> (not Linux).  That left the hardware architects looking for a way to
>> represent a new memory type (shadow stack) within the existing bits.
>> They chose to repurpose a lightly-used state: Write=0, Dirty=1.
>>
>> The reason it's lightly used is that Dirty=1 is normally set by hardware
>> and cannot normally be set by hardware on a Write=0 PTE.  Software must
>> normally be involved to create one of these PTEs, so software can simply
>> opt to not create them.
>>
>> In places where Linux normally creates Write=0, Dirty=1, it can use the
>> software-defined _PAGE_COW in place of the hardware _PAGE_DIRTY.  In other
>> words, whenever Linux needs to create Write=0, Dirty=1, it instead creates
>> Write=0, Cow=1, except for shadow stack, which is Write=0, Dirty=1.  This
>> clearly separates shadow stack from other data, and results in the
>> following:
>>
>> (a) A modified, copy-on-write (COW) page: (Write=0, Cow=1)
>> (b) A R/O page that has been COW'ed: (Write=0, Cow=1)
>>      The user page is in a R/O VMA, and get_user_pages() needs a writable
>>      copy.  The page fault handler creates a copy of the page and sets
>>      the new copy's PTE as Write=0 and Cow=1.
>> (c) A shadow stack PTE: (Write=0, Dirty=1)
>> (d) A shared shadow stack PTE: (Write=0, Cow=1)
>>      When a shadow stack page is being shared among processes (this happens
>>      at fork()), its PTE is made Dirty=0, so the next shadow stack access
>>      causes a fault, and the page is duplicated and Dirty=1 is set again.
>>      This is the COW equivalent for shadow stack pages, even though it's
>>      copy-on-access rather than copy-on-write.
>> (e) A page where the processor observed a Write=1 PTE, started a write, set
>>      Dirty=1, but then observed a Write=0 PTE.  That's possible today, but
>>      will not happen on processors that support shadow stack.
>>
>> Define _PAGE_COW and update pte_*() helpers and apply the same changes to
>> pmd and pud.
> 
> I still find this commit confusing mostly due to _PAGE_COW being 0
> without CET enabled. Shouldn't this just get changed universally? Why
> should this change depend on CET?
> 

For example, in...

static inline int pte_write(pte_t pte)
{
	if (cpu_feature_enabled(X86_FEATURE_SHSTK))
		return pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY);
	else
		return pte_flags(pte) & _PAGE_RW;
}

There are four cases:

(a) RW=1, Dirty=1 -> writable
(b) RW=1, Dirty=0 -> writable
(c) RW=0, Dirty=0 -> not writable
(d) RW=0, Dirty=1 -> shadow stack, or not-writable if !X86_FEATURE_SHSTK

Case (d) is ture only when shadow stack is enabled, otherwise it is not 
writable.  With shadow stack feature, the usual dirty, copy-on-write PTE 
becomes RW=0, Cow=1.

We can get this changed universally, but all usual dirty, copy-on-write 
PTEs need the Dirty/Cow swapping, always.  Is that desirable?

--
Yu-cheng

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