Re: [PATCH v8 11/27] x86/mm: Introduce _PAGE_DIRTY_SW

Dave Hansen
2019-08-13T23:02:24Z

> +#if defined(CONFIG_X86_INTEL_SHADOW_STACK_USER)
> +static inline pte_t pte_move_flags(pte_t pte, pteval_t from, pteval_t to)
> +{
> + if (pte_flags(pte) & from)
> + pte = pte_set_flags(pte_clear_flags(pte, from), to);
> + return pte;

Why is this conditional on the compile option and not a runtime check?

> +}
> +#else
> +static inline pte_t pte_move_flags(pte_t pte, pteval_t from, pteval_t to)
> +{
> + return pte;
> +}
> +#endif

Why do we need this function? It's not mentioned in the changelog or
commented.

> static inline pte_t pte_mkclean(pte_t pte)
> {
> - return pte_clear_flags(pte, _PAGE_DIRTY);
> + return pte_clear_flags(pte, _PAGE_DIRTY_BITS);
> }
>
> static inline pte_t pte_mkold(pte_t pte)
> @@ -322,6 +336,7 @@ static inline pte_t pte_mkold(pte_t pte)
>
> static inline pte_t pte_wrprotect(pte_t pte)
> {
> + pte = pte_move_flags(pte, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW);
> return pte_clear_flags(pte, _PAGE_RW);
> }

Please comment what this is doing and why.

> @@ -332,9 +347,24 @@ static inline pte_t pte_mkexec(pte_t pte)
>
> static inline pte_t pte_mkdirty(pte_t pte)
> {
> + pteval_t dirty = (!IS_ENABLED(CONFIG_X86_INTEL_SHADOW_STACK_USER) ||
> + pte_write(pte)) ? _PAGE_DIRTY_HW:_PAGE_DIRTY_SW;

This is *really* hard for me to read and parse. How about:

pte_t dirty = _PAGE_DIRTY_HW;

/*
* When Shadow Stacks are enabled, read-only PTEs can
* not have the hardware dirty bit set and must use
* the software bit.
*/
if (IS_ENABLED(CONFIG_X86_INTEL_SHADOW_STACK_USER) &&
!pte_write(pte))
dirty = _PAGE_DIRTY_SW;

> + return pte_set_flags(pte, dirty | _PAGE_SOFT_DIRTY);
> +}
> +
> +#ifdef CONFIG_ARCH_HAS_SHSTK
> +static inline pte_t pte_mkdirty_shstk(pte_t pte)
> +{
> + pte = pte_clear_flags(pte, _PAGE_DIRTY_SW);
> return pte_set_flags(pte, _PAGE_DIRTY_HW | _PAGE_SOFT_DIRTY);
> }

Why does the _PAGE_DIRTY_SW *HAVE* to be cleared on shstk pages?

> +static inline bool pte_dirty_hw(pte_t pte)
> +{
> + return pte_flags(pte) & _PAGE_DIRTY_HW;
> +}
> +#endif

Why are these #ifdef'd?

> static inline pte_t pte_mkyoung(pte_t pte)
> {
> return pte_set_flags(pte, _PAGE_ACCESSED);
> @@ -342,6 +372,7 @@ static inline pte_t pte_mkyoung(pte_t pte)
>
> static inline pte_t pte_mkwrite(pte_t pte)
> {
> + pte = pte_move_flags(pte, _PAGE_DIRTY_SW, _PAGE_DIRTY_HW);
> return pte_set_flags(pte, _PAGE_RW);
> }

It also isn't clear to me why this *must* move bits here. Its doubly
unclear why you would need to do this on systems when shadow stacks are
compiled in but disabled.

<snip>

Same comments for pmds and puds.

> -
> /* mprotect needs to preserve PAT bits when updating vm_page_prot */
> #define pgprot_modify pgprot_modify
> static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
> @@ -1178,6 +1254,19 @@ static inline int pmd_write(pmd_t pmd)
> return pmd_flags(pmd) & _PAGE_RW;
> }
>
> +static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
> +{
> + pmdval_t val = pmd_val(pmd), oldval = val;
> +
> + val &= _HPAGE_CHG_MASK;
> + val |= check_pgprot(newprot) & ~_HPAGE_CHG_MASK;
> + val = flip_protnone_guard(oldval, val, PHYSICAL_PMD_PAGE_MASK);
> + if ((pmd_write(pmd) && !(pgprot_val(newprot) & _PAGE_RW)))
> + return pmd_move_flags(__pmd(val), _PAGE_DIRTY_HW,
> + _PAGE_DIRTY_SW);
> + return __pmd(val);
> +}

Why was this function moved? This makes it really hard to review what
you changed

I'm going to stop reading this code now. It needs a lot more care and
feeding to make it reviewable. Please go back, double-check your
changelogs and flesh them out, then please try to make the code more
readable and understandable by commenting it.

Please take all of the compile-time checks and ask yourself whether they
need to be or *can* be runtime checks. Consider what the overhead is of
non-shadowstack systems running shadowstack-required code.

Please also reconcile the supervisor XSAVE portion of your patches with
the ones that Fenghua has been sending around. I've given quite a bit
of feedback to improve those. Please consolidate and agree on a common
set of patches with him.