Re: [PATCH v26 30/30] mm: Introduce PROT_SHADOW_STACK for shadow stack

Kirill A. Shutemov
2021-05-11T11:48:57Z

On Tue, Apr 27, 2021 at 01:43:15PM -0700, Yu-cheng Yu wrote:
> 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

Nit: you can put VM_SHADOW_STACK directly into VM_FLAGS_CLEAR. It's
already conditinal on the feature enabled and VM_NONE otherwise.

Up to you.

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

--
Kirill A. Shutemov