linux-kernel - Re: [PATCH v13 10/13] x86/sgx: Add sgx

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Date:   Tue, 4 Sep 2018 09:35:46 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     "Huang, Kai" <kai.huang@...el.com>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "suresh.b.siddha@...el.com" <suresh.b.siddha@...el.com>,
        "Ayoun, Serge" <serge.ayoun@...el.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "npmccallum@...hat.com" <npmccallum@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [PATCH v13 10/13] x86/sgx: Add sgx_einit() for initializing
 enclaves

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.

> 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.