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    Subject[PATCH v26 15/30] x86/mm: Update maybe_mkwrite() for shadow stack
    Date
    When serving a page fault, maybe_mkwrite() makes a PTE writable if its vma
    has VM_WRITE.

    A shadow stack vma has VM_SHADOW_STACK. Its PTEs have _PAGE_DIRTY, but not
    _PAGE_WRITE. In fork(), _PAGE_DIRTY is cleared to cause copy-on-write,
    and in the page fault handler, _PAGE_DIRTY is restored and the shadow stack
    page is writable again.

    Introduce an x86 version of maybe_mkwrite(), which sets proper PTE bits
    according to VM flags.

    Apply the same changes to maybe_pmd_mkwrite().

    Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Kees Cook <keescook@chromium.org>
    ---
    v24:
    - Instead of doing arch_maybe_mkwrite(), overwrite maybe*_mkwrite() with x86
    versions.
    - Change VM_SHSTK to VM_SHADOW_STACK.

    arch/x86/include/asm/pgtable.h | 6 ++++++
    arch/x86/mm/pgtable.c | 20 ++++++++++++++++++++
    include/linux/mm.h | 2 ++
    mm/huge_memory.c | 2 ++
    4 files changed, 30 insertions(+)

    diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
    index 46d9394b884f..da5dea417663 100644
    --- a/arch/x86/include/asm/pgtable.h
    +++ b/arch/x86/include/asm/pgtable.h
    @@ -308,6 +308,9 @@ static inline int pmd_trans_huge(pmd_t pmd)
    return (pmd_val(pmd) & (_PAGE_PSE|_PAGE_DEVMAP)) == _PAGE_PSE;
    }

    +#define maybe_pmd_mkwrite maybe_pmd_mkwrite
    +extern pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma);
    +
    #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
    static inline int pud_trans_huge(pud_t pud)
    {
    @@ -1686,6 +1689,9 @@ static inline bool arch_faults_on_old_pte(void)
    return false;
    }

    +#define maybe_mkwrite maybe_mkwrite
    +extern pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma);
    +
    #endif /* __ASSEMBLY__ */

    #endif /* _ASM_X86_PGTABLE_H */
    diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
    index f6a9e2e36642..e778dbbef3d8 100644
    --- a/arch/x86/mm/pgtable.c
    +++ b/arch/x86/mm/pgtable.c
    @@ -610,6 +610,26 @@ int pmdp_clear_flush_young(struct vm_area_struct *vma,
    }
    #endif

    +pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
    +{
    + if (likely(vma->vm_flags & VM_WRITE))
    + pte = pte_mkwrite(pte);
    + else if (likely(vma->vm_flags & VM_SHADOW_STACK))
    + pte = pte_mkwrite_shstk(pte);
    + return pte;
    +}
    +
    +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
    +pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
    +{
    + if (likely(vma->vm_flags & VM_WRITE))
    + pmd = pmd_mkwrite(pmd);
    + else if (likely(vma->vm_flags & VM_SHADOW_STACK))
    + pmd = pmd_mkwrite_shstk(pmd);
    + return pmd;
    +}
    +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
    +
    /**
    * reserve_top_address - reserves a hole in the top of kernel address space
    * @reserve - size of hole to reserve
    diff --git a/include/linux/mm.h b/include/linux/mm.h
    index 08282eb2f195..6ac9b3e9a865 100644
    --- a/include/linux/mm.h
    +++ b/include/linux/mm.h
    @@ -993,12 +993,14 @@ void free_compound_page(struct page *page);
    * pte_mkwrite. But get_user_pages can cause write faults for mappings
    * that do not have writing enabled, when used by access_process_vm.
    */
    +#ifndef maybe_mkwrite
    static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
    {
    if (likely(vma->vm_flags & VM_WRITE))
    pte = pte_mkwrite(pte);
    return pte;
    }
    +#endif

    vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page);
    void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr);
    diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    index ae907a9c2050..8203bd6ae4bd 100644
    --- a/mm/huge_memory.c
    +++ b/mm/huge_memory.c
    @@ -478,12 +478,14 @@ static int __init setup_transparent_hugepage(char *str)
    }
    __setup("transparent_hugepage=", setup_transparent_hugepage);

    +#ifndef maybe_pmd_mkwrite
    pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
    {
    if (likely(vma->vm_flags & VM_WRITE))
    pmd = pmd_mkwrite(pmd);
    return pmd;
    }
    +#endif

    #ifdef CONFIG_MEMCG
    static inline struct deferred_split *get_deferred_split_queue(struct page *page)
    --
    2.21.0
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