Re: [PATCH v4 10/39] x86/mm: Introduce _PAGE_COW

Borislav Petkov
2022-12-20T21:29:22Z

On Fri, Dec 02, 2022 at 04:35:37PM -0800, Rick Edgecombe wrote:
> There are six bits left available to software in the 64-bit PTE after
> consuming a bit for _PAGE_COW. No space is consumed in 32-bit kernels
> because shadow stacks are not enabled there.
>
> This is a prepratory patch. Changes to actually start marking _PAGE_COW

Unknown word [prepratory] in commit message.
Suggestions: ['preparatory',

> will follow once other pieces are in place.

And regardless, you don't really need this sentence at all, AFAICT.

...

> +/*
> + * Normally COW memory can result in Dirty=1,Write=0 PTs. But in the case
^^^

PTEs.

> + * of X86_FEATURE_USER_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

Unknown word [conventially] in comment.
Suggestions: ['conventionally', ...

> + * shadow stack compatible version of COW (Cow=1).
> + */
> +

^ Superfluous newline.

> +static inline pte_t pte_mkcow(pte_t pte)
> +{
> + if (!cpu_feature_enabled(X86_FEATURE_USER_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_USER_SHSTK kernels.

I'm guessing this "unnecessary" is supposed to mean that on kernels not
supporting shadow stack, a COW page uses the old bit flags?

I.e., Dirty=1,Write=0?

Might as well write it this way to be perfectly clear.

> + * See the _PAGE_COW definition for more details.
> + */
> + if (!cpu_feature_enabled(X86_FEATURE_USER_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

"Set the dirty bit.."

> + * cases.
> + */
> + pte = pte_set_flags(pte, _PAGE_DIRTY);
> + return pte_clear_flags(pte, _PAGE_COW);
> +}

Rest looks ok.

Thx.

--
Regards/Gruss,
Boris.