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

[Huang, Kai]

> -----Original Message-----
> From: Christopherson, Sean J
> Sent: Thursday, August 30, 2018 8:34 AM
> To: Huang, Kai <kai.huang@xxxxxxxxx>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>; platform-driver-
> x86@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx; nhorman@xxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; suresh.b.siddha@xxxxxxxxx; Ayoun,
> Serge <serge.ayoun@xxxxxxxxx>; hpa@xxxxxxxxx; npmccallum@xxxxxxxxxx;
> mingo@xxxxxxxxxx; linux-sgx@xxxxxxxxxxxxxxx; Hansen, Dave
> <dave.hansen@xxxxxxxxx>
> 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? 

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