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    Subject[PATCH v23 09/28] x86/mm: Introduce _PAGE_COW
    Date
    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.

    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.

    Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    ---
    arch/x86/include/asm/pgtable.h | 195 ++++++++++++++++++++++++---
    arch/x86/include/asm/pgtable_types.h | 42 +++++-
    2 files changed, 216 insertions(+), 21 deletions(-)

    diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
    index c1650d0af1b5..9c056d5815de 100644
    --- a/arch/x86/include/asm/pgtable.h
    +++ b/arch/x86/include/asm/pgtable.h
    @@ -121,11 +121,21 @@ 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;
    + /*
    + * A dirty PTE has Dirty=1 or Cow=1.
    + */
    + return pte_flags(pte) & _PAGE_DIRTY_BITS;
    }

    +static inline bool pte_shstk(pte_t pte)
    +{
    + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
    + return false;
    +
    + return (pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY)) == _PAGE_DIRTY;
    +}

    static inline u32 read_pkru(void)
    {
    @@ -160,9 +170,20 @@ 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)
    +{
    + /*
    + * A dirty PMD has Dirty=1 or Cow=1.
    + */
    + return pmd_flags(pmd) & _PAGE_DIRTY_BITS;
    +}
    +
    +static inline bool pmd_shstk(pmd_t pmd)
    {
    - return pmd_flags(pmd) & _PAGE_DIRTY;
    + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
    + return false;
    +
    + return (pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY)) == _PAGE_DIRTY;
    }

    static inline int pmd_young(pmd_t pmd)
    @@ -170,9 +191,12 @@ 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;
    + /*
    + * A dirty PUD has Dirty=1 or Cow=1.
    + */
    + return pud_flags(pud) & _PAGE_DIRTY_BITS;
    }

    static inline int pud_young(pud_t pud)
    @@ -182,13 +206,23 @@ static inline int pud_young(pud_t pud)

    static inline int pte_write(pte_t pte)
    {
    - return pte_flags(pte) & _PAGE_RW;
    + /*
    + * Shadow stack pages are always writable - but not by normal
    + * instructions, and only by shadow stack operations. Therefore,
    + * the W=0,D=1 test with pte_shstk().
    + */
    + return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte);
    }

    #define pmd_write pmd_write
    static inline int pmd_write(pmd_t pmd)
    {
    - return pmd_flags(pmd) & _PAGE_RW;
    + /*
    + * Shadow stack pages are always writable - but not by normal
    + * instructions, and only by shadow stack operations. Therefore,
    + * the W=0,D=1 test with pmd_shstk().
    + */
    + return (pmd_flags(pmd) & _PAGE_RW) || pmd_shstk(pmd);
    }

    #define pud_write pud_write
    @@ -326,6 +360,24 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
    return native_make_pte(v & ~clear);
    }

    +static inline pte_t pte_mkcow(pte_t pte)
    +{
    + if (!cpu_feature_enabled(X86_FEATURE_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)
    +{
    + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
    + return pte;
    +
    + pte = pte_set_flags(pte, _PAGE_DIRTY);
    + return pte_clear_flags(pte, _PAGE_COW);
    +}
    +
    #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
    static inline int pte_uffd_wp(pte_t pte)
    {
    @@ -345,7 +397,7 @@ 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)
    @@ -355,7 +407,16 @@ static inline pte_t pte_mkold(pte_t pte)

    static inline pte_t pte_wrprotect(pte_t pte)
    {
    - return pte_clear_flags(pte, _PAGE_RW);
    + pte = pte_clear_flags(pte, _PAGE_RW);
    +
    + /*
    + * 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 (pte_dirty(pte))
    + pte = pte_mkcow(pte);
    + return pte;
    }

    static inline pte_t pte_mkexec(pte_t pte)
    @@ -365,7 +426,18 @@ static inline pte_t pte_mkexec(pte_t pte)

    static inline pte_t pte_mkdirty(pte_t pte)
    {
    - return pte_set_flags(pte, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
    + 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)
    +{
    + return pte_clear_cow(pte);
    }

    static inline pte_t pte_mkyoung(pte_t pte)
    @@ -375,7 +447,12 @@ static inline pte_t pte_mkyoung(pte_t pte)

    static inline pte_t pte_mkwrite(pte_t pte)
    {
    - return pte_set_flags(pte, _PAGE_RW);
    + pte = pte_set_flags(pte, _PAGE_RW);
    +
    + if (pte_dirty(pte))
    + pte = pte_clear_cow(pte);
    +
    + return pte;
    }

    static inline pte_t pte_mkhuge(pte_t pte)
    @@ -422,6 +499,24 @@ static inline pmd_t pmd_clear_flags(pmd_t pmd, pmdval_t clear)
    return native_make_pmd(v & ~clear);
    }

    +static inline pmd_t pmd_mkcow(pmd_t pmd)
    +{
    + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
    + return pmd;
    +
    + pmd = pmd_clear_flags(pmd, _PAGE_DIRTY);
    + return pmd_set_flags(pmd, _PAGE_COW);
    +}
    +
    +static inline pmd_t pmd_clear_cow(pmd_t pmd)
    +{
    + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
    + return pmd;
    +
    + pmd = pmd_set_flags(pmd, _PAGE_DIRTY);
    + return pmd_clear_flags(pmd, _PAGE_COW);
    +}
    +
    #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
    static inline int pmd_uffd_wp(pmd_t pmd)
    {
    @@ -446,17 +541,36 @@ 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)
    {
    - return pmd_clear_flags(pmd, _PAGE_RW);
    + pmd = pmd_clear_flags(pmd, _PAGE_RW);
    + /*
    + * 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 (pmd_dirty(pmd))
    + pmd = pmd_mkcow(pmd);
    + return pmd;
    }

    static inline pmd_t pmd_mkdirty(pmd_t pmd)
    {
    - return pmd_set_flags(pmd, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
    + pmdval_t dirty = _PAGE_DIRTY;
    +
    + /* Avoid creating (HW)Dirty=1, Write=0 PMDs */
    + if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !pmd_write(pmd))
    + dirty = _PAGE_COW;
    +
    + return pmd_set_flags(pmd, dirty | _PAGE_SOFT_DIRTY);
    +}
    +
    +static inline pmd_t pmd_mkwrite_shstk(pmd_t pmd)
    +{
    + return pmd_clear_cow(pmd);
    }

    static inline pmd_t pmd_mkdevmap(pmd_t pmd)
    @@ -476,7 +590,11 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd)

    static inline pmd_t pmd_mkwrite(pmd_t pmd)
    {
    - return pmd_set_flags(pmd, _PAGE_RW);
    + pmd = pmd_set_flags(pmd, _PAGE_RW);
    +
    + if (pmd_dirty(pmd))
    + pmd = pmd_clear_cow(pmd);
    + return pmd;
    }

    static inline pud_t pud_set_flags(pud_t pud, pudval_t set)
    @@ -493,6 +611,24 @@ static inline pud_t pud_clear_flags(pud_t pud, pudval_t clear)
    return native_make_pud(v & ~clear);
    }

    +static inline pud_t pud_mkcow(pud_t pud)
    +{
    + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
    + return pud;
    +
    + pud = pud_clear_flags(pud, _PAGE_DIRTY);
    + return pud_set_flags(pud, _PAGE_COW);
    +}
    +
    +static inline pud_t pud_clear_cow(pud_t pud)
    +{
    + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
    + return pud;
    +
    + pud = pud_set_flags(pud, _PAGE_DIRTY);
    + return pud_clear_flags(pud, _PAGE_COW);
    +}
    +
    static inline pud_t pud_mkold(pud_t pud)
    {
    return pud_clear_flags(pud, _PAGE_ACCESSED);
    @@ -500,17 +636,32 @@ 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)
    {
    - return pud_clear_flags(pud, _PAGE_RW);
    + pud = pud_clear_flags(pud, _PAGE_RW);
    +
    + /*
    + * 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 (pud_dirty(pud))
    + pud = pud_mkcow(pud);
    + return pud;
    }

    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_write(pud))
    + dirty = _PAGE_COW;
    +
    + return pud_set_flags(pud, dirty | _PAGE_SOFT_DIRTY);
    }

    static inline pud_t pud_mkdevmap(pud_t pud)
    @@ -530,7 +681,11 @@ static inline pud_t pud_mkyoung(pud_t pud)

    static inline pud_t pud_mkwrite(pud_t pud)
    {
    - return pud_set_flags(pud, _PAGE_RW);
    + pud = pud_set_flags(pud, _PAGE_RW);
    +
    + if (pud_dirty(pud))
    + pud = pud_clear_cow(pud);
    + return pud;
    }

    #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
    diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
    index 9db61817dfff..38a1e8fe41c8 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
    \
     
     \ /
      Last update: 2021-03-16 16:13    [W:4.949 / U:0.176 seconds]
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