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

[Yu, Yu-cheng]

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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