Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW

[Jann Horn]

On Mon, Oct 3, 2022 at 11:36 PM Edgecombe, Rick P
<rick.p.edgecombe@xxxxxxxxx> wrote:
> On Mon, 2022-10-03 at 19:26 +0300, Kirill A . Shutemov wrote:
> > On Thu, Sep 29, 2022 at 03:29:07PM -0700, Rick Edgecombe wrote:
> > > +/*
> > > + * Normally the Dirty bit is used to denote COW memory on x86. But
> > > + * in the case of X86_FEATURE_SHSTK, the software COW bit is used,
> > > + * since the Dirty=1,Write=0 will result in the memory being
> > > treated
> > > + * as shaodw stack by the HW. So when creating COW memory, a
> > > software
> > > + * bit is used _PAGE_BIT_COW. The following functions pte_mkcow()
> > > and
> > > + * pte_clear_cow() take a PTE marked conventially COW (Dirty=1)
> > > and
> > > + * transition it to the shadow stack compatible version of COW
> > > (Cow=1).
> > > + */
> > > +
> > > +static inline pte_t pte_mkcow(pte_t pte)
> > > +{
> > > +     if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> > > +             return pte;
> > > +
> > > +     pte = pte_clear_flags(pte, _PAGE_DIRTY);
> > > +     return pte_set_flags(pte, _PAGE_COW);
> > > +}
> > > +
> > > +static inline pte_t pte_clear_cow(pte_t pte)
> > > +{
> > > +     /*
> > > +      * _PAGE_COW is unnecessary on !X86_FEATURE_SHSTK kernels.
> > > +      * See the _PAGE_COW definition for more details.
> > > +      */
> > > +     if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> > > +             return pte;
> > > +
> > > +     /*
> > > +      * PTE is getting copied-on-write, so it will be dirtied
> > > +      * if writable, or made shadow stack if shadow stack and
> > > +      * being copied on access. Set they dirty bit for both
> > > +      * cases.
> > > +      */
> > > +     pte = pte_set_flags(pte, _PAGE_DIRTY);
> > > +     return pte_clear_flags(pte, _PAGE_COW);
> > > +}
> >
> > These X86_FEATURE_SHSTK checks make me uneasy. Maybe use the
> > _PAGE_COW
> > logic for all machines with 64-bit entries. It will get you much more
> > coverage and more universal rules.
>
> Yes, I didn't like them either at first. The reasoning originally was
> that _PAGE_COW is a bit more work and it might show up for some
> benchmark.
>
> Looking at this again though, it is just a few more operations on
> memory that is already getting touched either way. It must be a very
> tiny amount of impact if any. I'm fine removing them. Having just one
> set of logic around this would make it easier to reason about.
>
> Dave, any thoughts on this?

But the rules wouldn't actually be universal - you'd still have to
look at X86_FEATURE_SHSTK in code that wants to figure out whether a
PTE is shadow stack (on a newer CPU) or readonly dirty (on an older
CPU that can set dirty bits on non-present PTEs), right?