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

[Andrew Cooper]

On 28/09/2020 21:42, Jarkko Sakkinen wrote:
> On Mon, Sep 28, 2020 at 05:44:35PM +0100, Andrew Cooper wrote:
>> On 28/09/2020 01:58, Jarkko Sakkinen wrote:
>>> On Fri, Sep 25, 2020 at 07:23:59PM +0100, Andrew Cooper wrote:
>>>> On 15/09/2020 12:28, Jarkko Sakkinen wrote:
>>>>> diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
>>>>> new file mode 100644
>>>>> index 000000000000..adbd59d41517
>>>>> --- /dev/null
>>>>> +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
>>>>> @@ -0,0 +1,157 @@
>>>>> +SYM_FUNC_START(__vdso_sgx_enter_enclave)
>>>>> <snip>
>>>>> +.Lretpoline:
>>>>> +	call	2f
>>>>> +1:	pause
>>>>> +	lfence
>>>>> +	jmp	1b
>>>>> +2:	mov	%rax, (%rsp)
>>>>> +	ret
>>>> I hate to throw further spanners in the work, but this is not compatible
>>>> with CET, and the user shadow stack work in progress.
>>> CET goes beyond my expertise. Can you describe, at least rudimentary,
>>> how this code is not compatible?
>> CET Shadow Stacks detect attacks which modify the return address on the
>> stack.
>>
>> Retpoline *is* a ROP gadget.  It really does modify the return address
>> on the stack, even if its purpose is defensive (vs Spectre v2) rather
>> than malicious.
> Aah. I get that, yes.
>
> Kernel is full of retpoline but I presume that ring-0 does not use CET.

No-one has implemented support CET-SS support for Linux itself yet, but
other kernels do have it working.

~Andrew