Re: [PATCH v13 10/13] x86/sgx: Add sgx_einit() for initializing enclaves

[Jarkko Sakkinen]

On Tue, Sep 04, 2018 at 09:35:46AM -0700, Sean Christopherson wrote:
> On Tue, Sep 04, 2018 at 06:30:21PM +0300, Jarkko Sakkinen wrote:
> > On Tue, Sep 04, 2018 at 07:54:51AM -0700, Sean Christopherson wrote:
> > > I don't see any value in trying to rule out specific causes of
> > > INVALID_TOKEN, but we should only retry EINIT if ret==INVALID_TOKEN
> > > and RDMSR(HASH0) != sgx_lepubkeyhash[0].  Only the first MSR needs to
> > > be checked for validity as they're a package deal, i.e. they'll all be
> > > valid or all be reset.  There shouldn't be a limit on retry attempts,
> > > e.g. the MSRs could theoretically be reset between WRMSR and EINIT.
> > 
> > Why is doing rdmsrs necessary? With the INVALID_TOKEN error we know we
> > are out-of-sync i.e. have been sleeping and then one just needs to do
> > wrmsrs.
> 
> As Kai mentioned, INVALID_TOKEN is returned for other reasons, e.g. a
> production enclave trying to use a debug token or reserved bits set in
> the token.  And in the KVM case, the hash and token are provided by
> the guest, so it's entirely possible the enclave/token is not signed
> with the key specified in the hash.  RDMSR is relatively inexpensive
> compared to the overall cost of EINIT.  Though of course EINIT failure
> isn't exactly a fast path, so I'm ok if you want to opt for simplicity
> and retry on INVALID_TOKEN without checking the MSRs, just make sure
> to add a comment indicating we're intentionally not checking the MSRs.

Great!

> > I think one retry should be enough given that VMM traps EINIT. One retry
> > is needed to take care of the guest itself (or host if we are running on
> > bare metal) having been in a sleep state.
> 
> Assuming we do RDMSR(hash0), that should be sufficient to prevent
> infinite retry and it protects against the MSRs being lost between
> WRMSR and EINIT during retry.  That being said, I'm ok retrying only
> once, especially if you want to omit the RDMSR.  Disabling preemption
> should prevent the kernel from suspending between WRMSR and EINIT,
> I'm just being paranoid.

But they are in the same preempt-disabled-region already?

/Jarkko