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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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