Re: [PATCH v7 14/41] x86/mm: Introduce _PAGE_SAVED_DIRTY

Borislav Petkov
2023-03-02T12:48:35Z

On Mon, Feb 27, 2023 at 02:29:30PM -0800, Rick Edgecombe wrote:
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 0646ad00178b..56b374d1bffb 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -21,7 +21,8 @@
> #define _PAGE_BIT_SOFTW2 10 /* " */
> #define _PAGE_BIT_SOFTW3 11 /* " */
> #define _PAGE_BIT_PAT_LARGE 12 /* On 2MB or 1GB pages */
> -#define _PAGE_BIT_SOFTW4 58 /* available for programmer */
> +#define _PAGE_BIT_SOFTW4 57 /* available for programmer */
> +#define _PAGE_BIT_SOFTW5 58 /* available for programmer */
> #define _PAGE_BIT_PKEY_BIT0 59 /* Protection Keys, bit 1/4 */
> #define _PAGE_BIT_PKEY_BIT1 60 /* Protection Keys, bit 2/4 */
> #define _PAGE_BIT_PKEY_BIT2 61 /* Protection Keys, bit 3/4 */
> @@ -34,6 +35,15 @@
> #define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty tracking */
> #define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4
>
> +/*
> + * Indicates a Saved Dirty bit page.
> + */
> +#ifdef CONFIG_X86_USER_SHADOW_STACK
> +#define _PAGE_BIT_SAVED_DIRTY _PAGE_BIT_SOFTW5 /* Saved Dirty bit */
> +#else
> +#define _PAGE_BIT_SAVED_DIRTY 0
> +#endif
> +
> /* If _PAGE_BIT_PRESENT is clear, we use these: */
> /* - if the user mapped it with PROT_NONE; pte_present gives true */
> #define _PAGE_BIT_PROTNONE _PAGE_BIT_GLOBAL
> @@ -117,6 +127,25 @@
> #define _PAGE_SOFTW4 (_AT(pteval_t, 0))
> #endif
>
> +/*
> + * The hardware requires shadow stack to be Write=0,Dirty=1. However,
> + * there are valid cases where the kernel might create read-only PTEs that
> + * are dirty (e.g., fork(), mprotect(), uffd-wp(), soft-dirty tracking). In
> + * this case, the _PAGE_SAVED_DIRTY bit is used instead of the HW-dirty bit,
> + * to avoid creating a wrong "shadow stack" PTEs. Such PTEs have
> + * (Write=0,SavedDirty=1,Dirty=0) set.
> + *
> + * Note that on processors without shadow stack support, the

.git/rebase-apply/patch:154: trailing whitespace.
* Note that on processors without shadow stack support, the
warning: 1 line adds whitespace errors.

Hm, apparently git checks for that too - not only trailing empty lines.

--
Regards/Gruss,
Boris.