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

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Aug 29, 2018 at 01:58:09PM -0700, Huang, Kai wrote:
> > -----Original Message-----
> > From: Christopherson, Sean J
> > Sent: Thursday, August 30, 2018 8:34 AM
> > To: Huang, Kai <kai.huang@intel.com>
> > Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>; platform-driver-
> > x86@vger.kernel.org; x86@kernel.org; nhorman@redhat.com; linux-
> > kernel@vger.kernel.org; tglx@linutronix.de; suresh.b.siddha@intel.com; Ayoun,
> > Serge <serge.ayoun@intel.com>; hpa@zytor.com; npmccallum@redhat.com;
> > mingo@redhat.com; linux-sgx@vger.kernel.org; Hansen, Dave
> > <dave.hansen@intel.com>
> > Subject: Re: [PATCH v13 10/13] x86/sgx: Add sgx_einit() for initializing enclaves
> > 
> > On Wed, Aug 29, 2018 at 12:33:54AM -0700, Huang, Kai wrote:
> > > [snip..]
> > >
> > > > > >
> > > > > > @@ -38,6 +39,18 @@ static LIST_HEAD(sgx_active_page_list);
> > > > > > static DEFINE_SPINLOCK(sgx_active_page_list_lock);
> > > > > >  static struct task_struct *ksgxswapd_tsk;  static
> > > > > > DECLARE_WAIT_QUEUE_HEAD(ksgxswapd_waitq);
> > > > > > +static struct notifier_block sgx_pm_notifier; static u64
> > > > > > +sgx_pm_cnt;
> > > > > > +
> > > > > > +/* The cache for the last known values of IA32_SGXLEPUBKEYHASHx
> > > > > > +MSRs
> > > > > > for each
> > > > > > + * CPU. The entries are initialized when they are first used by
> > > > > > sgx_einit().
> > > > > > + */
> > > > > > +struct sgx_lepubkeyhash {
> > > > > > +	u64 msrs[4];
> > > > > > +	u64 pm_cnt;
> > > > >
> > > > > May I ask why do we need pm_cnt here? In fact why do we need
> > > > > suspend staff (namely, sgx_pm_cnt above, and related code in this
> > > > > patch) here in this patch? From the patch commit message I don't
> > > > > see why we need PM staff here. Please give comment why you need PM
> > > > > staff, or you may consider to split the PM staff to another patch.
> > > >
> > > > Refining the commit message probably makes more sense because
> > > > without PM code sgx_einit() would be broken. The MSRs have been reset
> > after waking up.
> > > >
> > > > Some kind of counter is required to keep track of the power cycle.
> > > > When going to sleep the sgx_pm_cnt is increased. sgx_einit()
> > > > compares the current value of the global count to the value in the
> > > > cache entry to see whether we are in a new power cycle.
> > >
> > > You mean reset to Intel default? I think we can also just reset the
> > > cached MSR values on each power cycle, which would be simpler, IMHO?
> > 
> > Refresh my brain, does hardware reset the MSRs on a transition to S3 or lower?
> > 
> > > I think we definitely need some code to handle S3-S5, but should be in
> > > separate patches, since I think the major impact of S3-S5 is entire
> > > EPC being destroyed. I think keeping pm_cnt is not sufficient enough
> > > to handle such case?
> > > >
> > > > This brings up one question though: how do we deal with VM host going to
> > sleep?
> > > > VM guest would not be aware of this.
> > >
> > > IMO VM just gets "sudden loss of EPC" after suspend & resume in host.
> > > SGX driver and SDK should be able to handle "sudden loss of EPC", ie,
> > > co-working together to re-establish the missing enclaves.
> > >
> > > Actually supporting "sudden loss of EPC" is a requirement to support
> > > live migration of VM w/ SGX. Internally long time ago we had a
> > > discussion and the decision was we should support SGX live migration given
> > two facts:
> > >
> > > 1) losing platform-dependent is not important. For example, losing
> > > sealing key is not a problem, as we could get secrets provisioned
> > > again from remote. 2) Both windows & linux driver commit to support "sudden
> > loss of EPC".
> > >
> > > I don't think we have to support in very first upstream driver, but I
> > > think we need to support someday.
> > 
> > Actually, we can easily support this in the driver, at least for SGX1 hardware.
> 
> That's my guess too. Just want to check whether we are still on the same page :)
> 
> > SGX2 isn't difficult to handle, but we've intentionally postponed those patches
> > until SGX1 support is in mainline[1].
> > Accesses to the EPC after it is lost will cause faults.  Userspace EPC accesses, e.g.
> > ERESUME, will get a SIGSEGV that the process should interpret as an "I should
> > restart my enclave" event.  The SDK already does this.  In the driver, we just need
> > to be aware of this potential behavior and not freak out.  Specifically, SGX_INVD
> > needs to not WARN on faults that may have been due to a the EPC being nuked.
> > I think we can even remove the sgx_encl_pm_notifier() code altogether.
> 
> Possibly we still need to do some cleanup, ie, all structures of enclaves, upon resume? 

Not for functional reasons.  The driver will automatically do the
cleanup via SGX_INVD when it next accesses the enclave's pages and
takes a fault, e.g. during reclaim.  Proactively reclaiming the EPC
pages would probably affect performance, though not necessarily in
a good way.  And I think it would be a beneficial to get the driver
out of the suspend/hibernate/resume paths, e.g. zapping all enclaves
could noticeably impact suspend/resume latency.
 
> Anyway I am just guessing.
> 
> Thanks,
> -Kai
> 
> > 
> > [1] SGX1 hardware signals a #GP on an access to an invalid EPC page.
> >     SGX2 signals a #PF with the PF_SGX error code bit set.  This is
> >     problematic because the kernel looks at the PTEs for CR2 and sees
> >     nothing wrong, so it thinks it should just restart the
> >     instruction, leading to an infinite fault loop.  Resolving this
> >     is fairly straightforward, but a complete fix requires propagating
> >     PF_SGX down to the ENCLS fixup handler, which means plumbing the
> >     error code through the fixup handlers or smushing PF_SGX into
> >     trapnr.  Since there is a parallel effort to plumb the error code
> >     through the handlers, https://lkml.org/lkml/2018/8/6/924, we opted
> >     to do this in a separate series.
> > 
> > > Sean,
> > >
> > > Would you be able to comment here?
> > >
> > > >
> > > > I think the best measure would be to add a new parameter to
> > > > sgx_einit() that enforces update of the MSRs. The driver can then
> > > > set this parameter in the case when sgx_einit() returns
> > > > SGX_INVALID_LICENSE. This is coherent because the driver requires
> > > > writable MSRs. It would not be coherent to do it directly in the core because
> > KVM does not require writable MSRs.
> > >
> > > IMHO this is not required, as I mentioned above.
> > >
> > > And
> > > [snip...]
> > >
> > > Thanks,
> > > -Kai