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    Subject[PATCH v6 18/41] mm: Introduce VM_SHADOW_STACK for shadow stack memory
    Date
    From: Yu-cheng Yu <yu-cheng.yu@intel.com>

    The x86 Control-flow Enforcement Technology (CET) feature includes a new
    type of memory called shadow stack. This shadow stack memory has some
    unusual properties, which requires some core mm changes to function
    properly.

    A shadow stack PTE must be read-only and have _PAGE_DIRTY set. However,
    read-only and Dirty PTEs also exist for copy-on-write (COW) pages. These
    two cases are handled differently for page faults. Introduce
    VM_SHADOW_STACK to track shadow stack VMAs.

    Reviewed-by: Kees Cook <keescook@chromium.org>
    Tested-by: Pengfei Xu <pengfei.xu@intel.com>
    Tested-by: John Allen <john.allen@amd.com>
    Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
    Cc: Kees Cook <keescook@chromium.org>

    ---
    v6:
    - Add comment about VM_SHADOW_STACK not being allowed with VM_SHARED
    (David Hildenbrand)

    v3:
    - Drop arch specific change in arch_vma_name(). The memory can show as
    anonymous (Kirill)
    - Change CONFIG_ARCH_HAS_SHADOW_STACK to CONFIG_X86_USER_SHADOW_STACK
    in show_smap_vma_flags() (Boris)
    ---
    Documentation/filesystems/proc.rst | 1 +
    fs/proc/task_mmu.c | 3 +++
    include/linux/mm.h | 8 ++++++++
    3 files changed, 12 insertions(+)

    diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
    index e224b6d5b642..115843e8cce3 100644
    --- a/Documentation/filesystems/proc.rst
    +++ b/Documentation/filesystems/proc.rst
    @@ -564,6 +564,7 @@ encoded manner. The codes are the following:
    mt arm64 MTE allocation tags are enabled
    um userfaultfd missing tracking
    uw userfaultfd wr-protect tracking
    + ss shadow stack page
    == =======================================

    Note that there is no guarantee that every flag and associated mnemonic will
    diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
    index af1c49ae11b1..9e2cefe47749 100644
    --- a/fs/proc/task_mmu.c
    +++ b/fs/proc/task_mmu.c
    @@ -711,6 +711,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
    #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
    [ilog2(VM_UFFD_MINOR)] = "ui",
    #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
    +#ifdef CONFIG_X86_USER_SHADOW_STACK
    + [ilog2(VM_SHADOW_STACK)] = "ss",
    +#endif
    };
    size_t i;

    diff --git a/include/linux/mm.h b/include/linux/mm.h
    index e6f1789c8e69..76e0a09aeffe 100644
    --- a/include/linux/mm.h
    +++ b/include/linux/mm.h
    @@ -315,11 +315,13 @@ extern unsigned int kobjsize(const void *objp);
    #define VM_HIGH_ARCH_BIT_2 34 /* bit only usable on 64-bit architectures */
    #define VM_HIGH_ARCH_BIT_3 35 /* bit only usable on 64-bit architectures */
    #define VM_HIGH_ARCH_BIT_4 36 /* bit only usable on 64-bit architectures */
    +#define VM_HIGH_ARCH_BIT_5 37 /* bit only usable on 64-bit architectures */
    #define VM_HIGH_ARCH_0 BIT(VM_HIGH_ARCH_BIT_0)
    #define VM_HIGH_ARCH_1 BIT(VM_HIGH_ARCH_BIT_1)
    #define VM_HIGH_ARCH_2 BIT(VM_HIGH_ARCH_BIT_2)
    #define VM_HIGH_ARCH_3 BIT(VM_HIGH_ARCH_BIT_3)
    #define VM_HIGH_ARCH_4 BIT(VM_HIGH_ARCH_BIT_4)
    +#define VM_HIGH_ARCH_5 BIT(VM_HIGH_ARCH_BIT_5)
    #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */

    #ifdef CONFIG_ARCH_HAS_PKEYS
    @@ -335,6 +337,12 @@ extern unsigned int kobjsize(const void *objp);
    #endif
    #endif /* CONFIG_ARCH_HAS_PKEYS */

    +#ifdef CONFIG_X86_USER_SHADOW_STACK
    +# define VM_SHADOW_STACK VM_HIGH_ARCH_5 /* Should not be set with VM_SHARED */
    +#else
    +# define VM_SHADOW_STACK VM_NONE
    +#endif
    +
    #if defined(CONFIG_X86)
    # define VM_PAT VM_ARCH_1 /* PAT reserves whole VMA at once (x86) */
    #elif defined(CONFIG_PPC)
    --
    2.17.1
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