Re: [PATCH 11/35] x86/mm: Update pte_modify for _PAGE_COW

[Dave Hansen]

On 1/30/22 13:18, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> The read-only and Dirty PTE has been used to indicate copy-on-write pages.

Nit: This is another opportunity to use consistent terminology
     for these Write=0,Dirty=1 PTEs.

> However, newer x86 processors also regard a read-only and Dirty PTE as a
> shadow stack page.  In order to separate the two, the software-defined
> _PAGE_COW is created to replace _PAGE_DIRTY for the copy-on-write case, and
> pte_*() are updated.

The tense here is weird.  "_PAGE_COW is created" is present tense, but
it refers to something that happened earlier in the series.

> Pte_modify() changes a PTE to 'newprot', but it doesn't use the pte_*().

I'm not seeing a clear problem statement in there.  It looks something
like this to me:

	pte_modify() takes a "raw" pgprot_t which was not necessarily
	created with any of the existing PTE bit helpers.  That means
	that it can return a pte_t with Write=0,Dirty=1: a shadow stack
	PTE when it did not intend to create one.

But, this kinda looks like a hack to me.

It all boils down to _PAGE_CHG_MASK.  If pte_modify() can change the
bit's value, it is not included in _PAGE_CHG_MASK.  But, pte_modify()
*CAN* change the _PAGE_DIRTY value now.

Another way of saying it is that _PAGE_DIRTY is now a permission bit
(part-time, at least).


> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index a4a75e78a934..5c3886f6ccda 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -773,6 +773,23 @@ static inline pmd_t pmd_mkinvalid(pmd_t pmd)
>  
>  static inline u64 flip_protnone_guard(u64 oldval, u64 val, u64 mask);
>  
> +static inline pteval_t fixup_dirty_pte(pteval_t pteval)
> +{
> +	pte_t pte = __pte(pteval);
> +
> +	/*
> +	 * Fix up potential shadow stack page flags because the RO, Dirty
> +	 * PTE is special.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> +		if (pte_dirty(pte)) {
> +			pte = pte_mkclean(pte);
> +			pte = pte_mkdirty(pte);
> +		}
> +	}
> +	return pte_val(pte);
> +}
> +
>  static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>  {
>  	pteval_t val = pte_val(pte), oldval = val;
> @@ -783,16 +800,36 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>  	 */
>  	val &= _PAGE_CHG_MASK;
>  	val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
> +	val = fixup_dirty_pte(val);
>  	val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);
>  	return __pte(val);
>  }

Maybe something like this?  We can take _PAGE_DIRTY out of
_PAGE_CHG_MASK, then the p*_modify() functions look like this:

static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
{
        pteval_t val = pte_val(pte), oldval = val;
+	pte_t pte_result;

        /* Chop off any bits that might change with 'newprot':  */
        val &= _PAGE_CHG_MASK;
        val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
        val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);

+       pte_result = __pte(val);
+
+	if (pte_dirty(oldval))
+		pte_result = pte_mkdirty(pte_result));
+	
+	return pte_result;
}


This:

1. Makes logical sense: the dirty bit *IS* special in that it has to be
   logically preserved across permission changes.
2. Would work with or without shadow stacks.  That exact code would even
   work on a non-shadow-stack kernel
3. Doesn't introduce *any* new shadow-stack conditional code; the one
   already hidden in pte_mkdirty() is sufficient.
4. Avoids silly things like setting a bit and then immediately clearing
   it in a "fixup".
5. Removes the opaque "fixup" abstraction function.

That's way better if I do say so myself.