Re: [PATCH v25 30/30] mm: Introduce PROT_SHSTK for shadow stack

["Yu, Yu-cheng" <yu-cheng.yu@intel.com>]

On 4/25/2021 11:52 PM, Kirill A. Shutemov wrote:
> On Thu, Apr 15, 2021 at 03:14:19PM -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_SHSTK to mmap()/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_SHSTK to mmap()/mprotect() takes advantage of
> 
> Maybe PROT_SHADOW_STACK?
> 
>> existing APIs.  The x86-specific PROT_SHSTK 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>
>> ---
>> 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      | 59 +++++++++++++++++++++++++++++++-
>>   arch/x86/include/uapi/asm/mman.h |  1 +
>>   include/linux/mm.h               |  1 +
>>   3 files changed, 60 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/mman.h b/arch/x86/include/asm/mman.h
>> index 629f6c81263a..1821c179f35d 100644
>> --- a/arch/x86/include/asm/mman.h
>> +++ b/arch/x86/include/asm/mman.h

[...]

>> +
>> +#define arch_validate_prot arch_validate_prot
>> +
>> +static inline bool arch_validate_flags(unsigned long vm_flags, bool is_anon)
>> +{
>> +	if (vm_flags & VM_SHADOW_STACK) {
>> +		if ((vm_flags & VM_SHARED) || !is_anon)
> 
> VM_SHARED check is redundant. vma_is_anonymous() should be enough.
> Anonymous shared mappings would fail vma_is_anonymous().
>

Thanks for looking into this.  I will update and send another version.

Yu-cheng