Re: [PATCH v7 14/41] x86/mm: Introduce _PAGE_SAVED_DIRTY

Edgecombe, Rick P
2023-03-02T17:02:14Z

On Thu, 2023-03-02 at 13:48 +0100, Borislav Petkov wrote:
> On Mon, Feb 27, 2023 at 02:29:30PM -0800, Rick Edgecombe wrote:
> > diff --git a/arch/x86/include/asm/pgtable_types.h
> > b/arch/x86/include/asm/pgtable_types.h
> > index 0646ad00178b..56b374d1bffb 100644
> > --- a/arch/x86/include/asm/pgtable_types.h
> > +++ b/arch/x86/include/asm/pgtable_types.h
> > @@ -21,7 +21,8 @@
> > #define _PAGE_BIT_SOFTW2 10 /* " */
> > #define _PAGE_BIT_SOFTW3 11 /* " */
> > #define _PAGE_BIT_PAT_LARGE 12 /* On 2MB or 1GB pages */
> > -#define _PAGE_BIT_SOFTW4 58 /* available for programmer
> > */
> > +#define _PAGE_BIT_SOFTW4 57 /* available for programmer
> > */
> > +#define _PAGE_BIT_SOFTW5 58 /* available for programmer
> > */
> > #define _PAGE_BIT_PKEY_BIT0 59 /* Protection Keys, bit 1/4
> > */
> > #define _PAGE_BIT_PKEY_BIT1 60 /* Protection Keys, bit 2/4
> > */
> > #define _PAGE_BIT_PKEY_BIT2 61 /* Protection Keys, bit 3/4
> > */
> > @@ -34,6 +35,15 @@
> > #define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty
> > tracking */
> > #define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4
> >
> > +/*
> > + * Indicates a Saved Dirty bit page.
> > + */
> > +#ifdef CONFIG_X86_USER_SHADOW_STACK
> > +#define _PAGE_BIT_SAVED_DIRTY _PAGE_BIT_SOFTW5 /*
> > Saved Dirty bit */
> > +#else
> > +#define _PAGE_BIT_SAVED_DIRTY 0
> > +#endif
> > +
> > /* If _PAGE_BIT_PRESENT is clear, we use these: */
> > /* - if the user mapped it with PROT_NONE; pte_present gives true
> > */
> > #define _PAGE_BIT_PROTNONE _PAGE_BIT_GLOBAL
> > @@ -117,6 +127,25 @@
> > #define _PAGE_SOFTW4 (_AT(pteval_t, 0))
> > #endif
> >
> > +/*
> > + * The hardware requires shadow stack to be Write=0,Dirty=1.
> > However,
> > + * there are valid cases where the kernel might create read-only
> > PTEs that
> > + * are dirty (e.g., fork(), mprotect(), uffd-wp(), soft-dirty
> > tracking). In
> > + * this case, the _PAGE_SAVED_DIRTY bit is used instead of the HW-
> > dirty bit,
> > + * to avoid creating a wrong "shadow stack" PTEs. Such PTEs have
> > + * (Write=0,SavedDirty=1,Dirty=0) set.
> > + *
> > + * Note that on processors without shadow stack support, the
>
> .git/rebase-apply/patch:154: trailing whitespace.
> * Note that on processors without shadow stack support, the
> warning: 1 line adds whitespace errors.
>
> Hm, apparently git checks for that too - not only trailing empty
> lines.

Weird. And oops on the space. Just wondering how checkpatch missed
this. It didn't, just was in a pile of false positives on that patch
and I didn't notice it in there.