Re: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

[Dave Hansen]

On 9/28/20 11:12 AM, Andy Lutomirski wrote:
>> endbr64
>> /* Check if shadow stack is in use.  NB: R11 is the only usable
>>    scratch register for function calls.  */
>> xorl %r11d, %r11d
>> rdsspq %r11
>> testq %r11, %r11
>> jnz 3f
>> call 2f
>> 1:
>> pause
>> lfence
>> jmp 1b
>> 2:
>> mov %rax, (%rsp)
>> ret
>> 3:
>> /* Shadow stack is in use.  Make the indirect call.  */
>> call *%rax
>> ret
> What do we expect user programs to do on CET systems?  It would be
> nice if we could instead ALTERNATIVE this out if X86_FEATURE_SHSTK.

Shouldn't we just be able to use X86_FEATURE_RETPOLINE?

We probably need a mechanism to force X86_FEATURE_SHSTK and
X86_FEATURE_RETPOLINE to be mutually exclusive if we don't have one already.