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

Yu, Yu-cheng
2021-05-11T14:44:09Z

On 5/11/2021 4:48 AM, Kirill A. Shutemov wrote:
> 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>
>

Thanks!