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

On Tue, 2022-12-20 at 22:29 +0100, Borislav Petkov wrote:

On Fri, Dec 02, 2022 at 04:35:37PM -0800, Rick Edgecombe wrote:

quoted

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',

Sorry about all these spelling errors.

quoted

will follow once other pieces are in place.

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

...

Ok.

quoted

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

                                                        ^^^

PTEs.

Thanks.

quoted

+ * 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', ...

quoted

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

^ Superfluous newline.

Thanks.

quoted

+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.

Right, I can clarify.

quoted

+        * 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.."

Oof.

quoted

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

Rest looks ok.

Thanks.

Thx.