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

[Jarkko Sakkinen]

On Mon, Sep 03, 2018 at 03:53:24PM +0200, Jann Horn wrote:
> On Mon, Sep 3, 2018 at 3:33 PM Jarkko Sakkinen
> <jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:
> >
> > From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> >
> > Add a function to perform ENCLS(EINIT), which initializes an enclave,
> > which can be used by a driver for running enclaves and VMMs.
> >
> > Writing the LE hash MSRs is extraordinarily expensive, e.g. 3-4x slower
> > than normal MSRs, so we use a per-cpu cache to track the last known value
> > of the MSRs to avoid unnecessarily writing the MSRs with the current value.
> >
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Co-developed-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> [...]
> > +/**
> > + * sgx_einit - initialize an enclave
> > + * @sigstruct:         a pointer to the SIGSTRUCT
> > + * @token:             a pointer to the EINITTOKEN
> > + * @secs_page:         a pointer to the SECS EPC page
> > + * @lepubkeyhash:      the desired value for IA32_SGXLEPUBKEYHASHx MSRs
> > + *
> > + * Try to perform EINIT operation. If the MSRs are writable, they are updated
> > + * according to @lepubkeyhash.
> > + *
> > + * Return:
> > + *   0 on success,
> > + *   -errno on failure
> > + *   SGX error code if EINIT fails
> > + */
> > +int sgx_einit(struct sgx_sigstruct *sigstruct, struct sgx_einittoken *token,
> > +             struct sgx_epc_page *secs_page, u64 lepubkeyhash[4])
> > +{
> > +       struct sgx_lepubkeyhash __percpu *cache;
> > +       bool cache_valid;
> > +       int i, ret;
> > +
> > +       if (!sgx_lc_enabled)
> > +               return __einit(sigstruct, token, sgx_epc_addr(secs_page));
> > +
> > +       cache = per_cpu(sgx_lepubkeyhash_cache, smp_processor_id());
> 
> At this point, preemption must be off, because smp_processor_id() is
> called; I don't think it is off here? If you have hardware/emulation
> on which you can test this, you may want to test your patches with
> DEBUG_PREEMPT enabled.

Yeah, it really should. Thanks for spotting.

> 
> > +       if (!cache) {
> > +               cache = kzalloc(sizeof(struct sgx_lepubkeyhash), GFP_KERNEL);
> 
> But then here you do a GFP_KERNEL allocation, which can sleep.

Yes, of course this would need to be moved outside of the region where
pre-emption is disabled.

> Also: After "cache" has been allocated in this branch, when do you
> store the reference to it? As far as I can tell, you never write to
> sgx_lepubkeyhash_cache, and the allocation just leaks.

I have assignment in my local tree but for some reason it was not
squashed to this commit :-/

> 
> > +               if (!cache)
> > +                       return -ENOMEM;
> > +       }
> > +
> > +       cache_valid = cache->pm_cnt == sgx_pm_cnt;
> 
> The cache should probably not be treated as valid if it has just been
> created and only contains zeroes, right?

The name of the local variable is probably a bit misleading but also
cached values are compared in the loop.

> 
> > +       cache->pm_cnt = sgx_pm_cnt;
> 
> Can sgx_pm_cnt be modified concurrently? If so, please use at least
> READ_ONCE() to document that and prevent the compiler from doing weird
> stuff.

No it cannot.

> 
> > +       preempt_disable();
> 
> And here you turn off preemption, but it should already have been off?

Yes.

I think Sean's suggestion to update cache on SGX_INVALID_TOKEN is way to
go in this and instead of fixing this I'll change the code to use that
as a measure to update the cache.

/Jarkko