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

From: Jarkko Sakkinen <jarkko@kernel.org>
To: Kai Huang <kai.huang@intel.com>
Cc: Reinette Chatre <reinette.chatre@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	dave.hansen@linux.intel.com, linux-sgx@vger.kernel.org,
	haitao.huang@intel.com
Subject: Re: [RFC PATCH 1/4] x86/sgx: Do not free backing memory on ENCLS[ELDU] failure
Date: Wed, 11 May 2022 13:26:15 +0300	[thread overview]
Message-ID: <YnuPR3OW7xdEXnVF@kernel.org> (raw)
In-Reply-To: <bb918c317ebb96768365b38f00239013b67ad607.camel@intel.com>

On Tue, May 10, 2022 at 12:36:19PM +1200, Kai Huang wrote:
> 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.

It is :-) Yeah, it would be great if it could be made a bit more punctual!

BR, Jarkko