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

[Andrew Cooper]

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.

>> Whichever of these two large series lands first is going to inflict
>> fixing this problem on the other.
>>
>> As the vdso text is global (to a first approximation), it must not be a
>> retpoline if any other process is liable to want to use CET-SS.
> Why is that?

Because when CET-SS is enabled, the ret will suffer a #CP exception
(return address on the stack not matching the one recorded in the shadow
stack), which I presume/hope is wired into SIGSEGV.

~Andrew