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

Jarkko Sakkinen
2022-05-12T21:12:48Z

On Wed, May 11, 2022 at 01:29:59PM -0500, Haitao Huang wrote:
> Hi Jarkko
>
> On Wed, 11 May 2022 05:26:15 -0500, Jarkko Sakkinen <jarkko@kernel.org>
> wrote:
>
> > 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?
> > > >
>
>
> We discussed this internally, and agree this part of pseudo code needs be
> corrected/clarified.
>
> Here is what we think was going on when ELDU invoked with PCMD of all zeros:
> ELDU would check if the PCMD.SECINFO.FLAGS.PT is 0 which indicates that the
> page being loaded is a PT_SECS, and the PAGEINFO.SECS is not zero, then the
> instruction will #GP(0). Thus, ELDU is behaving correctly – it is an
> omission in the SDM pseudocode.
>
> The version checking code above also need be clarified because the VA slot
> would be cleared at this point and TMP_VER should be zero.
>
> We will follow up with the architect once he is back from sabbatical and
> make needed adjustment for SDM.

OK, great to hear, thank you!

BR, Jarkko