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    Subject[PATCH v26 30/30] mm: Introduce PROT_SHADOW_STACK for shadow stack
    Date
    There are three possible options to create a shadow stack allocation API:
    an arch_prctl, a new syscall, or adding PROT_SHADOW_STACK to mmap() and
    mprotect(). Each has its advantages and compromises.

    An arch_prctl() is the least intrusive. However, the existing x86
    arch_prctl() takes only two parameters. Multiple parameters must be
    passed in a memory buffer. There is a proposal to pass more parameters in
    registers [1], but no active discussion on that.

    A new syscall minimizes compatibility issues and offers an extensible frame
    work to other architectures, but this will likely result in some overlap of
    mmap()/mprotect().

    The introduction of PROT_SHADOW_STACK to mmap()/mprotect() takes advantage
    of existing APIs. The x86-specific PROT_SHADOW_STACK is translated to
    VM_SHADOW_STACK and a shadow stack mapping is created without reinventing
    the wheel. There are potential pitfalls though. The most obvious one
    would be using this as a bypass to shadow stack protection. However, the
    attacker would have to get to the syscall first.

    [1] https://lore.kernel.org/lkml/20200828121624.108243-1-hjl.tools@gmail.com/

    Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    Cc: Kees Cook <keescook@chromium.org>
    Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    ---
    v26:
    - Change PROT_SHSTK to PROT_SHADOW_STACK.
    - Remove (vm_flags & VM_SHARED) check, since it is covered by
    !vma_is_anonymous().

    v24:
    - Update arch_calc_vm_prot_bits(), leave PROT* checking to
    arch_validate_prot().
    - Update arch_validate_prot(), leave vma flags checking to
    arch_validate_flags().
    - Add arch_validate_flags().

    arch/x86/include/asm/mman.h | 60 +++++++++++++++++++++++++++++++-
    arch/x86/include/uapi/asm/mman.h | 2 ++
    include/linux/mm.h | 1 +
    3 files changed, 62 insertions(+), 1 deletion(-)

    diff --git a/arch/x86/include/asm/mman.h b/arch/x86/include/asm/mman.h
    index 629f6c81263a..fbb90f1b02c0 100644
    --- a/arch/x86/include/asm/mman.h
    +++ b/arch/x86/include/asm/mman.h
    @@ -20,11 +20,69 @@
    ((vm_flags) & VM_PKEY_BIT2 ? _PAGE_PKEY_BIT2 : 0) | \
    ((vm_flags) & VM_PKEY_BIT3 ? _PAGE_PKEY_BIT3 : 0))

    -#define arch_calc_vm_prot_bits(prot, key) ( \
    +#define pkey_vm_prot_bits(prot, key) ( \
    ((key) & 0x1 ? VM_PKEY_BIT0 : 0) | \
    ((key) & 0x2 ? VM_PKEY_BIT1 : 0) | \
    ((key) & 0x4 ? VM_PKEY_BIT2 : 0) | \
    ((key) & 0x8 ? VM_PKEY_BIT3 : 0))
    +#else
    +#define pkey_vm_prot_bits(prot, key) (0)
    #endif

    +static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
    + unsigned long pkey)
    +{
    + unsigned long vm_prot_bits = pkey_vm_prot_bits(prot, pkey);
    +
    + if (prot & PROT_SHADOW_STACK)
    + vm_prot_bits |= VM_SHADOW_STACK;
    +
    + return vm_prot_bits;
    +}
    +
    +#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
    +
    +#ifdef CONFIG_X86_SHADOW_STACK
    +static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
    +{
    + unsigned long valid = PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM |
    + PROT_SHADOW_STACK;
    +
    + if (prot & ~valid)
    + return false;
    +
    + if (prot & PROT_SHADOW_STACK) {
    + if (!current->thread.cet.shstk_size)
    + return false;
    +
    + /*
    + * A shadow stack mapping is indirectly writable by only
    + * the CALL and WRUSS instructions, but not other write
    + * instructions). PROT_SHADOW_STACK and PROT_WRITE are
    + * mutually exclusive.
    + */
    + if (prot & PROT_WRITE)
    + return false;
    + }
    +
    + return true;
    +}
    +
    +#define arch_validate_prot arch_validate_prot
    +
    +static inline bool arch_validate_flags(struct vm_area_struct *vma, unsigned long vm_flags)
    +{
    + /*
    + * Shadow stack must be anonymous and not shared.
    + */
    + if ((vm_flags & VM_SHADOW_STACK) && !vma_is_anonymous(vma))
    + return false;
    +
    + return true;
    +}
    +
    +#define arch_validate_flags(vma, vm_flags) arch_validate_flags(vma, vm_flags)
    +
    +#endif /* CONFIG_X86_SHADOW_STACK */
    +
    #endif /* _ASM_X86_MMAN_H */
    diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h
    index f28fa4acaeaf..4c36b263cf0a 100644
    --- a/arch/x86/include/uapi/asm/mman.h
    +++ b/arch/x86/include/uapi/asm/mman.h
    @@ -4,6 +4,8 @@

    #define MAP_32BIT 0x40 /* only give out 32bit addresses */

    +#define PROT_SHADOW_STACK 0x10 /* shadow stack pages */
    +
    #include <asm-generic/mman.h>

    #endif /* _UAPI_ASM_X86_MMAN_H */
    diff --git a/include/linux/mm.h b/include/linux/mm.h
    index 1ccec5cc399b..9a7652eea207 100644
    --- a/include/linux/mm.h
    +++ b/include/linux/mm.h
    @@ -342,6 +342,7 @@ extern unsigned int kobjsize(const void *objp);

    #if defined(CONFIG_X86)
    # define VM_PAT VM_ARCH_1 /* PAT reserves whole VMA at once (x86) */
    +# define VM_ARCH_CLEAR VM_SHADOW_STACK
    #elif defined(CONFIG_PPC)
    # define VM_SAO VM_ARCH_1 /* Strong Access Ordering (powerpc) */
    #elif defined(CONFIG_PARISC)
    --
    2.21.0
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