Re: [PATCH v19 08/25] x86/mm: Introduce _PAGE_COW

Kees Cook
2021-02-04T20:19:12Z

On Wed, Feb 03, 2021 at 02:55:30PM -0800, Yu-cheng Yu wrote:
> There is essentially no room left in the x86 hardware PTEs on some OSes
> (not Linux). That left the hardware architects looking for a way to
> represent a new memory type (shadow stack) within the existing bits.
> They chose to repurpose a lightly-used state: Write=0, Dirty=1.
>
> The reason it's lightly used is that Dirty=1 is normally set by hardware
> and cannot normally be set by hardware on a Write=0 PTE. Software must
> normally be involved to create one of these PTEs, so software can simply
> opt to not create them.
>
> In places where Linux normally creates Write=0, Dirty=1, it can use the
> software-defined _PAGE_COW in place of the hardware _PAGE_DIRTY. In other
> words, whenever Linux needs to create Write=0, Dirty=1, it instead creates
> Write=0, Cow=1, except for shadow stack, which is Write=0, Dirty=1. This
> clearly separates shadow stack from other data, and results in the
> following:
>
> (a) A modified, copy-on-write (COW) page: (Write=0, Cow=1)
> (b) A R/O page that has been COW'ed: (Write=0, Cow=1)
> The user page is in a R/O VMA, and get_user_pages() needs a writable
> copy. The page fault handler creates a copy of the page and sets
> the new copy's PTE as Write=0 and Cow=1.
> (c) A shadow stack PTE: (Write=0, Dirty=1)
> (d) A shared shadow stack PTE: (Write=0, Cow=1)
> When a shadow stack page is being shared among processes (this happens
> at fork()), its PTE is made Dirty=0, so the next shadow stack access
> causes a fault, and the page is duplicated and Dirty=1 is set again.
> This is the COW equivalent for shadow stack pages, even though it's
> copy-on-access rather than copy-on-write.
> (e) A page where the processor observed a Write=1 PTE, started a write, set
> Dirty=1, but then observed a Write=0 PTE. That's possible today, but
> will not happen on processors that support shadow stack.

What happens for "e" with/without CET? It sounds like direct writes to
such pages will be (correctly) rejected by the MMU?

>
> Define _PAGE_COW and update pte_*() helpers and apply the same changes to
> pmd and pud.
>
> After this, there are six free bits left in the 64-bit PTE, and no more
> free bits in the 32-bit PTE (except for PAE) and Shadow Stack is not
> implemented for the 32-bit kernel.

Are there selftests to validate this change?

I think it might be useful to more clearly describe what is considered
"dirty" and "writeable" in comments above the pte_helpers.

-Kees

>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> arch/x86/include/asm/pgtable.h | 125 ++++++++++++++++++++++++---
> arch/x86/include/asm/pgtable_types.h | 42 ++++++++-
> 2 files changed, 154 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index a02c67291cfc..4b0ec61510dc 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -121,9 +121,9 @@ extern pmdval_t early_pmd_flags;
> * The following only work if pte_present() is true.
> * Undefined behaviour if not..
> */
> -static inline int pte_dirty(pte_t pte)
> +static inline bool pte_dirty(pte_t pte)
> {
> - return pte_flags(pte) & _PAGE_DIRTY;
> + return pte_flags(pte) & _PAGE_DIRTY_BITS;
> }
>
>
> @@ -160,9 +160,9 @@ static inline int pte_young(pte_t pte)
> return pte_flags(pte) & _PAGE_ACCESSED;
> }
>
> -static inline int pmd_dirty(pmd_t pmd)
> +static inline bool pmd_dirty(pmd_t pmd)
> {
> - return pmd_flags(pmd) & _PAGE_DIRTY;
> + return pmd_flags(pmd) & _PAGE_DIRTY_BITS;
> }
>
> static inline int pmd_young(pmd_t pmd)
> @@ -170,9 +170,9 @@ static inline int pmd_young(pmd_t pmd)
> return pmd_flags(pmd) & _PAGE_ACCESSED;
> }
>
> -static inline int pud_dirty(pud_t pud)
> +static inline bool pud_dirty(pud_t pud)
> {
> - return pud_flags(pud) & _PAGE_DIRTY;
> + return pud_flags(pud) & _PAGE_DIRTY_BITS;
> }
>
> static inline int pud_young(pud_t pud)
> @@ -182,7 +182,14 @@ static inline int pud_young(pud_t pud)
>
> static inline int pte_write(pte_t pte)
> {
> - return pte_flags(pte) & _PAGE_RW;
> + /*
> + * If _PAGE_DIRTY is set, the PTE must either have _PAGE_RW or be
> + * a shadow stack PTE, which is logically writable.
> + */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK))
> + return pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY);
> + else
> + return pte_flags(pte) & _PAGE_RW;
> }
>
> static inline int pte_huge(pte_t pte)
> @@ -333,7 +340,7 @@ static inline pte_t pte_clear_uffd_wp(pte_t pte)
>
> 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)
> @@ -343,6 +350,18 @@ static inline pte_t pte_mkold(pte_t pte)
>
> static inline pte_t pte_wrprotect(pte_t pte)
> {
> + /*
> + * Blindly clearing _PAGE_RW might accidentally create
> + * a shadow stack PTE (RW=0, Dirty=1). Move the hardware
> + * dirty value to the software bit.
> + */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> + if (pte_flags(pte) & _PAGE_DIRTY) {
> + pte = pte_clear_flags(pte, _PAGE_DIRTY);
> + pte = pte_set_flags(pte, _PAGE_COW);
> + }
> + }
> +
> return pte_clear_flags(pte, _PAGE_RW);
> }
>
> @@ -353,6 +372,18 @@ static inline pte_t pte_mkexec(pte_t pte)
>
> static inline pte_t pte_mkdirty(pte_t pte)
> {
> + pteval_t dirty = _PAGE_DIRTY;
> +
> + /* Avoid creating (HW)Dirty=1, Write=0 PTEs */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !pte_write(pte))
> + dirty = _PAGE_COW;
> +
> + return pte_set_flags(pte, dirty | _PAGE_SOFT_DIRTY);
> +}
> +
> +static inline pte_t pte_mkwrite_shstk(pte_t pte)
> +{
> + pte = pte_clear_flags(pte, _PAGE_COW);
> return pte_set_flags(pte, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
> }
>
> @@ -363,6 +394,13 @@ static inline pte_t pte_mkyoung(pte_t pte)
>
> static inline pte_t pte_mkwrite(pte_t pte)
> {
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> + if (pte_flags(pte) & _PAGE_COW) {
> + pte = pte_clear_flags(pte, _PAGE_COW);
> + pte = pte_set_flags(pte, _PAGE_DIRTY);
> + }
> + }
> +
> return pte_set_flags(pte, _PAGE_RW);
> }
>
> @@ -434,16 +472,40 @@ static inline pmd_t pmd_mkold(pmd_t pmd)
>
> static inline pmd_t pmd_mkclean(pmd_t pmd)
> {
> - return pmd_clear_flags(pmd, _PAGE_DIRTY);
> + return pmd_clear_flags(pmd, _PAGE_DIRTY_BITS);
> }
>
> static inline pmd_t pmd_wrprotect(pmd_t pmd)
> {
> + /*
> + * Blindly clearing _PAGE_RW might accidentally create
> + * a shadow stack PMD (RW=0, Dirty=1). Move the hardware
> + * dirty value to the software bit.
> + */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> + if (pmd_flags(pmd) & _PAGE_DIRTY) {
> + pmd = pmd_clear_flags(pmd, _PAGE_DIRTY);
> + pmd = pmd_set_flags(pmd, _PAGE_COW);
> + }
> + }
> +
> return pmd_clear_flags(pmd, _PAGE_RW);
> }
>
> static inline pmd_t pmd_mkdirty(pmd_t pmd)
> {
> + pmdval_t dirty = _PAGE_DIRTY;
> +
> + /* Avoid creating (HW)Dirty=1, Write=0 PMDs */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pmd_flags(pmd) & _PAGE_RW))
> + dirty = _PAGE_COW;
> +
> + return pmd_set_flags(pmd, dirty | _PAGE_SOFT_DIRTY);
> +}
> +
> +static inline pmd_t pmd_mkwrite_shstk(pmd_t pmd)
> +{
> + pmd = pmd_clear_flags(pmd, _PAGE_COW);
> return pmd_set_flags(pmd, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
> }
>
> @@ -464,6 +526,13 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd)
>
> static inline pmd_t pmd_mkwrite(pmd_t pmd)
> {
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> + if (pmd_flags(pmd) & _PAGE_COW) {
> + pmd = pmd_clear_flags(pmd, _PAGE_COW);
> + pmd = pmd_set_flags(pmd, _PAGE_DIRTY);
> + }
> + }
> +
> return pmd_set_flags(pmd, _PAGE_RW);
> }
>
> @@ -488,17 +557,35 @@ static inline pud_t pud_mkold(pud_t pud)
>
> static inline pud_t pud_mkclean(pud_t pud)
> {
> - return pud_clear_flags(pud, _PAGE_DIRTY);
> + return pud_clear_flags(pud, _PAGE_DIRTY_BITS);
> }
>
> static inline pud_t pud_wrprotect(pud_t pud)
> {
> + /*
> + * Blindly clearing _PAGE_RW might accidentally create
> + * a shadow stack PUD (RW=0, Dirty=1). Move the hardware
> + * dirty value to the software bit.
> + */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> + if (pud_flags(pud) & _PAGE_DIRTY) {
> + pud = pud_clear_flags(pud, _PAGE_DIRTY);
> + pud = pud_set_flags(pud, _PAGE_COW);
> + }
> + }
> +
> return pud_clear_flags(pud, _PAGE_RW);
> }
>
> static inline pud_t pud_mkdirty(pud_t pud)
> {
> - return pud_set_flags(pud, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
> + pudval_t dirty = _PAGE_DIRTY;
> +
> + /* Avoid creating (HW)Dirty=1, Write=0 PUDs */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pud_flags(pud) & _PAGE_RW))
> + dirty = _PAGE_COW;
> +
> + return pud_set_flags(pud, dirty | _PAGE_SOFT_DIRTY);
> }
>
> static inline pud_t pud_mkdevmap(pud_t pud)
> @@ -518,6 +605,13 @@ static inline pud_t pud_mkyoung(pud_t pud)
>
> static inline pud_t pud_mkwrite(pud_t pud)
> {
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> + if (pud_flags(pud) & _PAGE_COW) {
> + pud = pud_clear_flags(pud, _PAGE_COW);
> + pud = pud_set_flags(pud, _PAGE_DIRTY);
> + }
> + }
> +
> return pud_set_flags(pud, _PAGE_RW);
> }
>
> @@ -1131,7 +1225,14 @@ extern int pmdp_clear_flush_young(struct vm_area_struct *vma,
> #define pmd_write pmd_write
> static inline int pmd_write(pmd_t pmd)
> {
> - return pmd_flags(pmd) & _PAGE_RW;
> + /*
> + * If _PAGE_DIRTY is set, then the PMD must either have _PAGE_RW or
> + * be a shadow stack PMD, which is logically writable.
> + */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK))
> + return pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY);
> + else
> + return pmd_flags(pmd) & _PAGE_RW;
> }
>
> #define __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index b8b79d618bbc..437d7ff0ae80 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -23,7 +23,8 @@
> #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 */
> @@ -36,6 +37,15 @@
> #define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty tracking */
> #define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4
>
> +/*
> + * Indicates a copy-on-write page.
> + */
> +#ifdef CONFIG_X86_CET
> +#define _PAGE_BIT_COW _PAGE_BIT_SOFTW5 /* copy-on-write */
> +#else
> +#define _PAGE_BIT_COW 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 +127,36 @@
> #define _PAGE_DEVMAP (_AT(pteval_t, 0))
> #endif
>
> +/*
> + * The hardware requires shadow stack to be read-only and Dirty.
> + * _PAGE_COW is a software-only bit used to separate copy-on-write PTEs
> + * from shadow stack PTEs:
> + * (a) A modified, copy-on-write (COW) page: (Write=0, Cow=1)
> + * (b) A R/O page that has been COW'ed: (Write=0, Cow=1)
> + * The user page is in a R/O VMA, and get_user_pages() needs a
> + * writable copy. The page fault handler creates a copy of the page
> + * and sets the new copy's PTE as Write=0, Cow=1.
> + * (c) A shadow stack PTE: (Write=0, Dirty=1)
> + * (d) A shared (copy-on-access) shadow stack PTE: (Write=0, Cow=1)
> + * When a shadow stack page is being shared among processes (this
> + * happens at fork()), its PTE is cleared of _PAGE_DIRTY, so the next
> + * shadow stack access causes a fault, and the page is duplicated and
> + * _PAGE_DIRTY is set again. This is the COW equivalent for shadow
> + * stack pages, even though it's copy-on-access rather than
> + * copy-on-write.
> + * (e) A page where the processor observed a Write=1 PTE, started a write,
> + * set Dirty=1, but then observed a Write=0 PTE (changed by another
> + * thread). That's possible today, but will not happen on processors
> + * that support shadow stack.
> + */
> +#ifdef CONFIG_X86_CET
> +#define _PAGE_COW (_AT(pteval_t, 1) << _PAGE_BIT_COW)
> +#else
> +#define _PAGE_COW (_AT(pteval_t, 0))
> +#endif
> +
> +#define _PAGE_DIRTY_BITS (_PAGE_DIRTY | _PAGE_COW)
> +
> #define _PAGE_PROTNONE (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)
>
> /*
> --
> 2.21.0
>

--
Kees Cook