Re: [PATCH v33 11/21] x86/sgx: Linux Enclave Driver

Jarkko Sakkinen
2020-07-07T03:30:02Z

On Mon, Jul 06, 2020 at 06:38:47PM -0700, Sean Christopherson wrote:
> On Sat, Jul 04, 2020 at 04:43:49AM +0300, Jarkko Sakkinen wrote:
> > On Mon, Jun 29, 2020 at 08:27:19AM -0700, Sean Christopherson wrote:
> > > On Sat, Jun 27, 2020 at 07:43:35PM +0200, Borislav Petkov wrote:
> > > > And you could do similar sanity checks in the other ioctl functions.
> > >
> > > Ya, as above, SGX_ENCL_INITIALIZED can be checked here.
> > >
> > > SGX_ENCL_DEAD is actually already checked in in the top level sgx_ioctl(),
> > > i.e. the check in sgx_encl_add_page() can technically be flat out dropped.
> > >
> > > I say "technically" because I'm a bit torn over SGX_ENCL_DEAD; encl->lock
> > > must be held to SGX_ENCL_DEAD (the page fault and reclaim flows rely on
> > > this), but as it stands today only ioctl() paths (guarded by SGX_ENCL_IOCTL)
> > > and sgx_release() (makes the ioctls() unreachable) set SGX_ENCL_DEAD.
> > >
> > > So it's safe to check SGX_ENCL_DEAD from ioctl() context without holding
> > > encl->lock, at least in the current code base, but it feels weird/sketchy.
> > >
> > > In the end I don't think I have a strong opinion. Removing the technically
> > > unnecessary DEAD check in sgx_encl_add_page() is the simplest change, so it
> > > may make sense to do that and nothing more for initial upstreaming. Long
> > > term, I fully expect we'll add paths that set SGX_ENCL_DEAD outside of
> > > ioctl() context, e.g. to handle EPC OOM, but it wouldn't be a bad thing to
> > > have a standalone commit in a future series to add DEAD checks (under
> > > encl->lock) in the ADD and INIT flows.
> >
> > AFAIK nonne of th ioctl's should not need SGX_ENCL_DEAD check.
>
> I can't tell if the double negative was intended, but I took a peek at your
> current master and see that you removed the SGX_ENCL_DEAD check in
> sgx_ioctl(). That check needs to stay, e.g. if EEXTEND fails we absolutely
> need to prevent any further operations on the enclave.
>
> The above was calling out that additional checks on SGX_ENCL_DEAD after
> acquiring encl->lock are not necessary because SGX_ENCL_DEAD can only be
> set when the ioctls() are no longer reachable or from within an ioctl(),
> which provides exclusivity via SGX_ENCL_IOCTL.

Got it.

/Jarkko