Re: [RFC PATCH 1/4] x86/sgx: Do not free backing memory on ENCLS[ELDU] failure

Kai Huang
2022-05-10T00:36:29Z

On Mon, 2022-05-09 at 10:17 -0700, Reinette Chatre wrote:
> Hi Jarkko,
>
> On 5/7/2022 10:25 AM, Jarkko Sakkinen wrote:
> > On Thu, Apr 28, 2022 at 04:49:00PM -0700, Reinette Chatre wrote:
> > > > I also looked a little deeper at this transient failure problem. The
> > > > ELDU documentation also mentions a possible error code of:
> > > >
> > > > SGX_EPC_PAGE_CONFLICT
> > > >
> > > > It *looks* like there can be conflicts on the SECS page as well as the
> > > > EPC page being explicitly accessed. Is that a possible problem here?
> > >
> > > I went down this path myself. SGX_EPC_PAGE_CONFLICT is an error code
> > > supported by newer ELDUC - the ELDU used in current code would indeed
> > > #GP in this case. The SDM text describing ELDUC as "This leaf function
> > > behaves like ELDU but with improved conflict handling for oversubscription"
> > > really does seem relevant to the test that triggers this issue.
> > >
> > > I stopped pursuing this because from what I understand if
> > > SGX_EPC_PAGE_CONFLICT is encountered with commit 08999b2489b4 ("x86/sgx:
> > > Free backing memory after faulting the enclave page") then it should
> > > also be encountered without it. The issue is not present with
> > > 08999b2489b4 ("x86/sgx: Free backing memory after faulting the
> > > enclave page") removed. I am thus currently investigating based on
> > > the assumption that the #GP is encountered because of MAC
> > > verification problem. I may be wrong here also and need more information
> > > since the SDM documents two seemingly related errors:
> > > #GP(0) -> If the instruction fails to verify MAC.
> > > SGX_MAC_COMPARE_FAIL -> If the MAC check fails.
> >
> > This part puzzles me in the pseudo-code.
> >
> > The version is read first:
> >
> > TMP_VER := DS:RDX[63:0];
> >
> > Then there's MAC calculation, comparison, and finally this check:
> >
> > (* Check version before committing *)
> > IF (DS:RDX ≠ 0)
> > THEN #GP(0);
> > ELSE
> > DS:RDX := TMP_VER;
> > FI;
> >
> > For me it is a mystery what does zero the slot and in what condition
> > it would be non-zero. Perhaps the #GP refers anyway to this check?
>
> RDX contains the VA slot information and that appears to be correct
> in these scenarios. The issue is the PCMD data pointed to by the
> PAGEINFO.PCMD (link to PAGEINFO found in RBX) is all zeroes.
>
> There are two scenarios under which the PCMD data could be zeroes. They
> are documented in:
> https://lore.kernel.org/linux-sgx/8157fa40-8d02-8819-e1b6-fd2d8863fb56@intel.com/
> https://lore.kernel.org/linux-sgx/da387afc-e666-45d0-1e99-066d8c4aab03@intel.com/
>
> I understand that context may be lost by pointing you to various emails
> in this thread - I'll wrap up all learnings when I submit the new version
> of this series today.
>

Hi Reinette,

Regardless the root cause of this problem, I agree with Jarkko above pseudo-code
in the spec is quite confusing. I can try to get it clarified from Intel
internally if you want.

--
Thanks,
-Kai