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

Dave Hansen
2022-02-09T18:00:37Z

On 1/30/22 13:18, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> 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.