Re: [PATCH v22 02/24] x86/cpufeatures: x86/msr: Intel SGX Launch Control hardware bits

[Borislav Petkov]

On Tue, Sep 24, 2019 at 01:22:10PM -0700, Sean Christopherson wrote:
> The approach we chose (patch 04, which we were discussing) is to disable
> SGX if SGX_LE_WR is not set, i.e. disallow SGX unless the hash MSRs exist
> and are fully writable.

Hmm, so I see

+       if (!(fc & FEATURE_CONTROL_SGX_LE_WR)) {
+               pr_info_once("sgx: The launch control MSRs are not writable\n");
+               goto err_msrs_rdonly;

which clears only X86_FEATURE_SGX_LC but leaves the other three feature
bits set?!

If you'd want to disable SGX then you'd need to jump to the
err_unsupported label and get rid of the err_msrs_rdonly one.

Or am I missing something?

> WRMSR will #GP if FEATURE_CONTROL is locked (bit 0), e.g. attempting to
> set SGX_LE_WR will trap if FEATURE_CONTROL was locked by BIOS.

Right.

> And conversely, the various enable bits in FEATURE_CONTROL don't
> take effect until FEATURE_CONTROL is locked, e.g. the LE hash MSRs
> aren't writable if FEATURE_CONTROL is unlocked, regardless of whether
> SGX_LE_WR is set.

Ok. We want them writable.

> Sadly, because FEATURE_CONTROL must be locked to fully enable SGX, the
> reality is that any BIOS that supports SGX will lock FEATURE_CONTROL.

That's fine. The question is, would OEMs leave the hash MSRs writable?

If, as you say above, we clear SGX feature bit - not only
X86_FEATURE_SGX_LC - when the MSRs are not writable, then we're fine.
Platforms sticking their own hash in there won't be supported but I
guess their aim is not to be supported in Linux anyway.

> That's the status quo today as well since VMX (and SMX/TXT) is also
> enabled via FEATURE_CONTROL.  KVM does have logic to enable VMX and lock
> FEATURE_CONTROL if the MSR isn't locked, but AIUI that exists only to work
> with old BIOSes.
> 
> If we want to support setting and locking FEATURE_CONTROL in the extremely
> unlikely scenario that BIOS left it unlocked, the proper change would be

I wouldn't be too surprised if this happened. BIOS is very inventive.

> One note on Launch Control that isn't covered in the SDM: the LE hash
> MSRs can also be written before SGX is activated.  SGX activation must
> occur before FEATURE_CONTROL is locked, meaning BIOS can set the LE
> hash MSRs to a non-intel and then lock FEATURE_CONTROL with SGX_LE_WR=0.

This is exactly what I'm afraid of. The OEM vendors locking this down.

> Heh, why stop at 4?  12_EBX, 12_1_ECX and 12_1_EDX are effectively feature
> leafs as well, although the kernel can ignore them for the most part.

Yeah, we're mentally prepared for the feature bit space explosion. :)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette