linux-kernel - Re: [PATCH v2 2/2] x86/sgx: Implement EUPDATESVN and opportunistically call it during first EPC page alloc

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Message-ID: <Z_TEzsYnCxXoEhi0@kernel.org>
Date: Tue, 8 Apr 2025 09:40:14 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: "Reshetova, Elena" <elena.reshetova@...el.com>,
	"Hansen, Dave" <dave.hansen@...el.com>,
	"linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
	"Scarlata, Vincent R" <vincent.r.scarlata@...el.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"Annapurve, Vishal" <vannapurve@...gle.com>,
	"bondarn@...gle.com" <bondarn@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>,
	"Aktas, Erdem" <erdemaktas@...gle.com>,
	"Cai, Chong" <chongc@...gle.com>,
	"Raynor, Scott" <scott.raynor@...el.com>,
	"dionnaglaze@...gle.com" <dionnaglaze@...gle.com>
Subject: Re: [PATCH v2 2/2] x86/sgx: Implement EUPDATESVN and
 opportunistically call it during first EPC page alloc

On Tue, Apr 08, 2025 at 12:06:32AM +0000, Huang, Kai wrote:
> On Mon, 2025-04-07 at 08:23 +0000, Reshetova, Elena wrote:
> > > On Fri, Apr 04, 2025 at 06:53:17AM +0000, Reshetova, Elena wrote:
> > > > > On Wed, Apr 02, 2025 at 01:11:25PM +0000, Reshetova, Elena wrote:
> > > > > > > > current SGX kernel code does not handle such errors in any other
> > > way
> > > > > > > > than notifying that operation failed for other ENCLS leaves. So, I don't
> > > > > > > > see why ENCLS[EUPDATESVN] should be different from existing
> > > > > behaviour?
> > > > > > > 
> > > > > > > While not disagreeing fully (it depends on call site), in some
> > > > > > > situations it is more difficult to take more preventive actions.
> > > > > > > 
> > > > > > > This is a situation where we know that there are *zero* EPC pages in
> > > > > > > traffic so it is relatively easy to stop the madness, isn't it?
> > > > > > > 
> > > > > > > I guess the best action would be make sgx_alloc_epc_page() return
> > > > > > > consistently -ENOMEM, if the unexpected happens.
> > > > > > 
> > > > > > But this would be very misleading imo. We do have memory, even page
> > > > > > allocation might function as normal in EPC, the only thing that is broken
> > > > > > can be EUPDATESVN functionality. Returning -ENOMEM in this case
> > > seems
> > > > > > wrong.
> > > > > 
> > > > > This makes it not misleading at all:
> > > > > 
> > > > > 	pr_err("EUPDATESVN: unknown error %d\n", ret);
> > > > > 
> > > > > Since hardware should never return this, it indicates a kernel bug.
> > > > 
> > > > OK, so you propose in this case to print the above message, sgx_updatesvn
> > > > returning an error, and then NULL from __sgx_alloc_epc_page_from_node
> > > and
> > > > the __sgx_alloc_epc_page returning -ENOMEM after an iteration over
> > > > a whole set of numa nodes given that we will keep getting the unknown
> > > error
> > > > on each node upon trying to do an allocation from each one?
> > > 
> > > I'd disable ioctl's in this case and return -ENOMEM. It's a cheap sanity
> > > check. Should not ever happen, but if e.g., a new kernel patch breaks
> > > anything, it could help catching issues.
> > > 
> > > We are talking here about situation that is never expected to happen so I
> > > don't think it is too heavy hammer here. Here it makes sense because not
> > > much effort is required to implement the counter-measures.
> > 
> > OK, but does it really make sense to explicitly disable ioctls? 
> > Note that everything *in practice* will be disabled simply because not a single page
> > anymore can be allocated from EPC since we are getting -ENOMEM on EPC
> > page allocation. Also, note that any approach we chose should be symmetrical
> > to SGX virtualization side also, which doesn´t use ioctls at all. Simply returning
> > -ENOMEM for page allocation in EPC seems like a correct symmetrical solution
> > that would work for both nativel enclaves and EPC pages allocated for VMs.
> > And nothing would  be able to proceed creating/managing enclaves at this point. 
> > 
> 
> Right, failing ioctls() doesn't cover SGX virtualization.  If we ever want to
> fail, we should fail the EPC allocation.

"I guess the best action would be make sgx_alloc_epc_page() return
 consistently -ENOMEM, if the unexpected happens." -me

> 
> Btw, for the unknown error, and any other errors which should not happen,
> couldn't we use the ENCLS_WARN()?  AFAICT there are already cases that we are
> using ENCLS_WARN() for those "impossible-to-happen-errors".
> 
> E.g., in __sgx_encl_extend():
> 
> 	        ret = __eextend(sgx_get_epc_virt_addr(encl->secs.epc_page),
>                                 sgx_get_epc_virt_addr(epc_page) + offset);
>                 if (ret) {
>                         if (encls_failed(ret))
>                                 ENCLS_WARN(ret, "EEXTEND");
>    
>                         return -EIO;
>                 }

BR, Jarkko