Messages in this thread | | | Date | Tue, 20 Dec 2022 22:29:13 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v4 10/39] x86/mm: Introduce _PAGE_COW |
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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.
https://people.kernel.org/tglx/notes-about-netiquette
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