Re: [PATCH v21 08/26] x86/mm: Introduce _PAGE_COW

Borislav Petkov
2021-03-01T15:53:42Z

On Wed, Feb 17, 2021 at 02:27:12PM -0800, Yu-cheng Yu wrote:
> @@ -182,7 +206,7 @@ static inline int pud_young(pud_t pud)
>
> static inline int pte_write(pte_t pte)
> {
> - return pte_flags(pte) & _PAGE_RW;

Put here a comment along the lines of:

/*
* Shadow stack pages are always writable - but not by normal
* instructions but only by shadow stack operations. Therefore, the
* W=0, D=1 test.
*/

to make it clear what this means.

> + return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte);
> }
>
> static inline int pte_huge(pte_t pte)
> @@ -314,6 +338,24 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
> return native_make_pte(v & ~clear);
> }
>
> +static inline pte_t pte_make_cow(pte_t pte)

pte_mkcow like the rest of the "pte_mkX" functions.

And below too, for the other newly added pXd_make_* helpers.

> static inline pmd_t pmd_mkdirty(pmd_t pmd)
> {
> - return pmd_set_flags(pmd, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
> + pmdval_t dirty = _PAGE_DIRTY;
> +
> + /* Avoid creating (HW)Dirty=1, Write=0 PMDs */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pmd_flags(pmd) & _PAGE_RW))

!(pmd_write(pmd))

> + dirty = _PAGE_COW;
> +
> + return pmd_set_flags(pmd, dirty | _PAGE_SOFT_DIRTY);
> +}

...

> static inline pud_t pud_mkdirty(pud_t pud)
> {
> - return pud_set_flags(pud, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
> + pudval_t dirty = _PAGE_DIRTY;
> +
> + /* Avoid creating (HW)Dirty=1, Write=0 PUDs */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pud_flags(pud) & _PAGE_RW))

!(pud_write(pud))

--
Regards/Gruss,
Boris.