Re: [RFC PATCH v9 09/27] x86/mm: Introduce _PAGE_DIRTY_SW

From: Kees Cook
Date: Tue Feb 25 2020 - 15:13:12 EST

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On Wed, Feb 05, 2020 at 10:19:17AM -0800, Yu-cheng Yu wrote:
> When Shadow Stack (SHSTK) is introduced, a R/O and Dirty PTE exists in the
> following cases:
>
> (a) A modified, copy-on-write (COW) page;
> (b) A R/O page that has been COW'ed;
> (c) A SHSTK page.
>
> To separate non-SHSTK memory from SHSTK, introduce a spare bit of the
> 64-bit PTE as _PAGE_BIT_DIRTY_SW and use that for case (a) and (b).
> This results in the following possible settings:
>
> Modified PTE: (R/W + DIRTY_HW)
> Modified and COW PTE: (R/O + DIRTY_SW)
> R/O PTE COW'ed: (R/O + DIRTY_SW)
> SHSTK PTE: (R/O + DIRTY_HW)
> SHSTK shared PTE[1]: (R/O + DIRTY_SW)
> SHSTK PTE COW'ed: (R/O + DIRTY_HW)
>
> [1] When a SHSTK page is being shared among threads, its PTE is cleared of
> _PAGE_DIRTY_HW, so the next SHSTK access causes a fault, and the page
> is duplicated and _PAGE_DIRTY_HW is set again.
>
> With this, in pte_wrprotect(), if SHSTK is active, use _PAGE_DIRTY_SW for
> the Dirty bit, and in pte_mkwrite() use _PAGE_DIRTY_HW. The same changes
> apply to pmd and pud.
>
> When this patch is applied, there are six free bits left in the 64-bit PTE.
> There are no more free bits in the 32-bit PTE (except for PAE) and SHSTK is
> not implemented for the 32-bit kernel.
>
> v9:
> - Remove pte_move_flags() etc. and put the logic directly in
> pte_wrprotect()/pte_mkwrite() etc.
> - Change compile-time conditionals to run-time checks.
> - Split out pte_modify()/pmd_modify() to a new patch.
> - Update comments.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
> arch/x86/include/asm/pgtable.h | 111 ++++++++++++++++++++++++---
> arch/x86/include/asm/pgtable_types.h | 31 +++++++-
> 2 files changed, 131 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index ab50d25f9afc..62aeb118bc36 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -120,9 +120,9 @@ 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_HW;
> + return pte_flags(pte) & _PAGE_DIRTY_BITS;
> }
>
>
> @@ -159,9 +159,9 @@ 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_HW;
> + return pmd_flags(pmd) & _PAGE_DIRTY_BITS;
> }
>
> static inline int pmd_young(pmd_t pmd)
> @@ -169,9 +169,9 @@ 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_HW;
> + return pud_flags(pud) & _PAGE_DIRTY_BITS;
> }
>
> static inline int pud_young(pud_t pud)
> @@ -312,7 +312,7 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
>
> static inline pte_t pte_mkclean(pte_t pte)
> {
> - return pte_clear_flags(pte, _PAGE_DIRTY_HW);
> + return pte_clear_flags(pte, _PAGE_DIRTY_BITS);
> }
>
> static inline pte_t pte_mkold(pte_t pte)
> @@ -322,6 +322,17 @@ static inline pte_t pte_mkold(pte_t pte)
>
> static inline pte_t pte_wrprotect(pte_t pte)
> {
> + /*
> + * Use _PAGE_DIRTY_SW on a R/O PTE to set it apart from
> + * a Shadow Stack PTE, which is R/O + _PAGE_DIRTY_HW.
> + */
> + if (static_cpu_has(X86_FEATURE_SHSTK)) {
> + if (pte_flags(pte) & _PAGE_DIRTY_HW) {
> + pte = pte_clear_flags(pte, _PAGE_DIRTY_HW);
> + pte = pte_set_flags(pte, _PAGE_DIRTY_SW);
> + }
> + }
> +
> return pte_clear_flags(pte, _PAGE_RW);
> }
>
> @@ -332,9 +343,25 @@ static inline pte_t pte_mkexec(pte_t pte)
>
> static inline pte_t pte_mkdirty(pte_t pte)
> {
> + pteval_t dirty = _PAGE_DIRTY_HW;
> +
> + if (static_cpu_has(X86_FEATURE_SHSTK) && !pte_write(pte))
> + dirty = _PAGE_DIRTY_SW;
> +
> + return pte_set_flags(pte, dirty | _PAGE_SOFT_DIRTY);
> +}
> +
> +static inline pte_t pte_mkdirty_shstk(pte_t pte)
> +{
> + pte = pte_clear_flags(pte, _PAGE_DIRTY_SW);
> return pte_set_flags(pte, _PAGE_DIRTY_HW | _PAGE_SOFT_DIRTY);
> }
>
> +static inline bool pte_dirty_hw(pte_t pte)
> +{
> + return pte_flags(pte) & _PAGE_DIRTY_HW;
> +}
> +
> static inline pte_t pte_mkyoung(pte_t pte)
> {
> return pte_set_flags(pte, _PAGE_ACCESSED);
> @@ -342,6 +369,13 @@ static inline pte_t pte_mkyoung(pte_t pte)
>
> static inline pte_t pte_mkwrite(pte_t pte)
> {
> + if (static_cpu_has(X86_FEATURE_SHSTK)) {
> + if (pte_flags(pte) & _PAGE_DIRTY_SW) {
> + pte = pte_clear_flags(pte, _PAGE_DIRTY_SW);
> + pte = pte_set_flags(pte, _PAGE_DIRTY_HW);
> + }
> + }
> +
> return pte_set_flags(pte, _PAGE_RW);
> }
>
> @@ -396,19 +430,46 @@ 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_HW);
> + return pmd_clear_flags(pmd, _PAGE_DIRTY_BITS);
> }
>
> static inline pmd_t pmd_wrprotect(pmd_t pmd)
> {
> + /*
> + * Use _PAGE_DIRTY_SW on a R/O PMD to set it apart from
> + * a Shadow Stack PTE, which is R/O + _PAGE_DIRTY_HW.
> + */
> + if (static_cpu_has(X86_FEATURE_SHSTK)) {
> + if (pmd_flags(pmd) & _PAGE_DIRTY_HW) {
> + pmd = pmd_clear_flags(pmd, _PAGE_DIRTY_HW);
> + pmd = pmd_set_flags(pmd, _PAGE_DIRTY_SW);
> + }
> + }
> +
> return pmd_clear_flags(pmd, _PAGE_RW);
> }
>
> static inline pmd_t pmd_mkdirty(pmd_t pmd)
> {
> + pmdval_t dirty = _PAGE_DIRTY_HW;
> +
> + if (static_cpu_has(X86_FEATURE_SHSTK) && !(pmd_flags(pmd) & _PAGE_RW))
> + dirty = _PAGE_DIRTY_SW;
> +
> + return pmd_set_flags(pmd, dirty | _PAGE_SOFT_DIRTY);
> +}
> +
> +static inline pmd_t pmd_mkdirty_shstk(pmd_t pmd)
> +{
> + pmd = pmd_clear_flags(pmd, _PAGE_DIRTY_SW);
> return pmd_set_flags(pmd, _PAGE_DIRTY_HW | _PAGE_SOFT_DIRTY);
> }
>
> +static inline bool pmd_dirty_hw(pmd_t pmd)
> +{
> + return pmd_flags(pmd) & _PAGE_DIRTY_HW;
> +}
> +
> static inline pmd_t pmd_mkdevmap(pmd_t pmd)
> {
> return pmd_set_flags(pmd, _PAGE_DEVMAP);
> @@ -426,6 +487,13 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd)
>
> static inline pmd_t pmd_mkwrite(pmd_t pmd)
> {
> + if (static_cpu_has(X86_FEATURE_SHSTK)) {
> + if (pmd_flags(pmd) & _PAGE_DIRTY_SW) {
> + pmd = pmd_clear_flags(pmd, _PAGE_DIRTY_SW);
> + pmd = pmd_set_flags(pmd, _PAGE_DIRTY_HW);
> + }
> + }
> +
> return pmd_set_flags(pmd, _PAGE_RW);
> }
>
> @@ -450,17 +518,33 @@ 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_HW);
> + return pud_clear_flags(pud, _PAGE_DIRTY_BITS);
> }
>
> static inline pud_t pud_wrprotect(pud_t pud)
> {
> + /*
> + * Use _PAGE_DIRTY_SW on a R/O PUD to set it apart from
> + * a Shadow Stack PTE, which is R/O + _PAGE_DIRTY_HW.
> + */
> + if (static_cpu_has(X86_FEATURE_SHSTK)) {
> + if (pud_flags(pud) & _PAGE_DIRTY_HW) {
> + pud = pud_clear_flags(pud, _PAGE_DIRTY_HW);
> + pud = pud_set_flags(pud, _PAGE_DIRTY_SW);
> + }
> + }
> +
> return pud_clear_flags(pud, _PAGE_RW);
> }
>
> static inline pud_t pud_mkdirty(pud_t pud)
> {
> - return pud_set_flags(pud, _PAGE_DIRTY_HW | _PAGE_SOFT_DIRTY);
> + pudval_t dirty = _PAGE_DIRTY_HW;
> +
> + if (static_cpu_has(X86_FEATURE_SHSTK) && !(pud_flags(pud) & _PAGE_RW))
> + dirty = _PAGE_DIRTY_SW;
> +
> + return pud_set_flags(pud, dirty | _PAGE_SOFT_DIRTY);
> }
>
> static inline pud_t pud_mkdevmap(pud_t pud)
> @@ -480,6 +564,13 @@ static inline pud_t pud_mkyoung(pud_t pud)
>
> static inline pud_t pud_mkwrite(pud_t pud)
> {
> + if (static_cpu_has(X86_FEATURE_SHSTK)) {
> + if (pud_flags(pud) & _PAGE_DIRTY_SW) {
> + pud = pud_clear_flags(pud, _PAGE_DIRTY_SW);
> + pud = pud_set_flags(pud, _PAGE_DIRTY_HW);
> + }
> + }
> +
> return pud_set_flags(pud, _PAGE_RW);
> }
>
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index e647e3c75578..826823df917f 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 */
> @@ -35,6 +36,12 @@
> #define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty tracking */
> #define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4
>
> +/*
> + * This bit indicates a copy-on-write page, and is different from
> + * _PAGE_BIT_SOFT_DIRTY, which tracks which pages a task writes to.
> + */
> +#define _PAGE_BIT_DIRTY_SW _PAGE_BIT_SOFTW5 /* was written to */
> +
> /* 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
> @@ -108,6 +115,28 @@
> #define _PAGE_DEVMAP (_AT(pteval_t, 0))
> #endif
>
> +/* A R/O and dirty PTE exists in the following cases:
> + * (a) A modified, copy-on-write (COW) page;
> + * (b) A R/O page that has been COW'ed;
> + * (c) A SHSTK page.
> + * _PAGE_DIRTY_SW is used to separate case (c) from others.
> + * This results in the following settings:
> + *
> + * Modified PTE: (R/W + DIRTY_HW)
> + * Modified and COW PTE: (R/O + DIRTY_SW)
> + * R/O PTE COW'ed: (R/O + DIRTY_SW)
> + * SHSTK PTE: (R/O + DIRTY_HW)
> + * SHSTK PTE COW'ed: (R/O + DIRTY_HW)
> + * SHSTK PTE being shared among threads: (R/O + DIRTY_SW)
> + */
> +#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> +#define _PAGE_DIRTY_SW (_AT(pteval_t, 1) << _PAGE_BIT_DIRTY_SW)
> +#else
> +#define _PAGE_DIRTY_SW (_AT(pteval_t, 0))
> +#endif
> +
> +#define _PAGE_DIRTY_BITS (_PAGE_DIRTY_HW | _PAGE_DIRTY_SW)
> +
> #define _PAGE_PROTNONE (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)
>
> #define _PAGE_TABLE_NOENC (_PAGE_PRESENT | _PAGE_RW | _PAGE_USER |\
> --
> 2.21.0
>

--
Kees Cook

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