Date:   Tue, 16 Mar 2021 12:50:50 +1300
From:   Kai Huang <kai.huang@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org,
        x86@...nel.org, linux-sgx@...r.kernel.org,
        linux-kernel@...r.kernel.org, luto@...nel.org,
        dave.hansen@...el.com, rick.p.edgecombe@...el.com,
        haitao.huang@...el.com, pbonzini@...hat.com, bp@...en8.de,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com
Subject: Re: [PATCH v3 03/25] x86/sgx: Wipe out EREMOVE from
 sgx_free_epc_page()

On Tue, 16 Mar 2021 00:59:31 +0200 Jarkko Sakkinen wrote:
> On Tue, Mar 16, 2021 at 09:29:34AM +1300, Kai Huang wrote:
> > On Mon, 15 Mar 2021 15:19:32 +0200 Jarkko Sakkinen wrote:
> > > On Mon, Mar 15, 2021 at 03:18:16PM +0200, Jarkko Sakkinen wrote:
> > > > On Mon, Mar 15, 2021 at 08:12:36PM +1300, Kai Huang wrote:
> > > > > On Sat, 13 Mar 2021 12:45:53 +0200 Jarkko Sakkinen wrote:
> > > > > > On Fri, Mar 12, 2021 at 01:21:54PM -0800, Sean Christopherson wrote:
> > > > > > > On Thu, Mar 11, 2021, Kai Huang wrote:
> > > > > > > > From: Jarkko Sakkinen <jarkko@...nel.org>
> > > > > > > > 
> > > > > > > > EREMOVE takes a page and removes any association between that page and
> > > > > > > > an enclave.  It must be run on a page before it can be added into
> > > > > > > > another enclave.  Currently, EREMOVE is run as part of pages being freed
> > > > > > > > into the SGX page allocator.  It is not expected to fail.
> > > > > > > > 
> > > > > > > > KVM does not track how guest pages are used, which means that SGX
> > > > > > > > virtualization use of EREMOVE might fail.
> > > > > > > > 
> > > > > > > > Break out the EREMOVE call from the SGX page allocator.  This will allow
> > > > > > > > the SGX virtualization code to use the allocator directly.  (SGX/KVM
> > > > > > > > will also introduce a more permissive EREMOVE helper).
> > > > > > > > 
> > > > > > > > Implement original sgx_free_epc_page() as sgx_encl_free_epc_page() to be
> > > > > > > > more specific that it is used to free EPC page assigned to one enclave.
> > > > > > > > Print an error message when EREMOVE fails to explicitly call out EPC
> > > > > > > > page is leaked, and requires machine reboot to get leaked pages back.
> > > > > > > > 
> > > > > > > > Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
> > > > > > > > Co-developed-by: Kai Huang <kai.huang@...el.com>
> > > > > > > > Acked-by: Jarkko Sakkinen <jarkko@...nel.org>
> > > > > > > > Signed-off-by: Kai Huang <kai.huang@...el.com>
> > > > > > > > ---
> > > > > > > > v2->v3:
> > > > > > > > 
> > > > > > > >  - Fixed bug during copy/paste which results in SECS page and va pages are not
> > > > > > > >    correctly freed in sgx_encl_release() (sorry for the mistake).
> > > > > > > >  - Added Jarkko's Acked-by.
> > > > > > > 
> > > > > > > That Acked-by should either be dropped or moved above Co-developed-by to make
> > > > > > > checkpatch happy.
> > > > > > > 
> > > > > > > Reviewed-by: Sean Christopherson <seanjc@...gle.com>
> > > > > > 
> > > > > > Oops, my bad. Yup, ack should be removed.
> > > > > > 
> > > > > > /Jarkko
> > > > > 
> > > > > Hi Jarkko,
> > > > > 
> > > > > Your reply of your concern of this patch to the cover-letter
> > > > > 
> > > > > https://lore.kernel.org/lkml/YEkJXu262YDa8ZaK@kernel.org/
> > > > > 
> > > > > reminds me to do more sanity check of whether removing EREMOVE in
> > > > > sgx_free_epc_page() will impact other code path or not, and I think
> > > > > sgx_encl_release() is not the only place should be changed:
> > > > > 
> > > > > - sgx_encl_shrink() needs to call sgx_encl_free_epc_page(), since when this is
> > > > > called, the VA page can be already valid -- there are other failures can
> > > > > trigger sgx_encl_shrink().
> > > > 
> > > > You right about this, good catch.
> > > > 
> > > > Shrink needs to always do EREMOVE as grow has done EPA, which changes
> > > > EPC page state.
> > > > 
> > > > > - sgx_encl_add_page() should call sgx_encl_free_epc_page() in "err_out_free:"
> > > > > label, since the EPC page can be already valid when error happened, i.e. when
> > > > > EEXTEND fails.
> > > > 
> > > > Yes, correct, good work!
> > > > 
> > > > > Other places should be OK per my check, but I'd prefer to just replacing all
> > > > > sgx_free_epc_page() call sites in driver with sgx_encl_free_epc_page(), with
> > > > > one exception: sgx_alloc_va_page(), which calls sgx_free_epc_page() when EPA
> > > > > fails, in which case EREMOVE is not required for sure.
> > > > 
> > > > I would not unless they require it.
> > > > 
> > > > > Your idea, please?
> > > > > 
> > > > > Btw, introducing a driver wrapper of sgx_free_epc_page() does make sense to me,
> > > > > because virtualization has a counterpart in sgx/virt.c too.
> > > > 
> > > > It does make sense to use sgx_free_epc_page() everywhere where it's
> > > > the right thing to call and here's why.
> > > > 
> > > > If there is some unrelated regression that causes EPC page not get
> > > > uninitialized when it actually should, doing extra EREMOVE could mask
> > > > those bugs. I.e. it can postpone a failure, which can make a bug harder
> > > > to backtrace.
> > > > 
> > > 
> > > I.e. even though it is true that for correctly working code extra EREMOVE
> > > is nil functionality, it could change semantics for buggy code.
> > 
> > Thanks for feedback. Sorry I am not sure if I understand you. So if we don't
> > want to bring functionality change, we need to replace sgx_free_epc_page() in
> > all call sites with sgx_encl_free_epc_page(). To me for this patch only, it's
> > better not to bring any functional change, so I intend to replace all (I now
> > consider even leaving sgx_alloc_va_page() out is not good idea in *this*
> > patch). 
> > 
> > Or do you just want to replace sgx_free_epc_page() with
> > sgx_encl_free_epc_page() in sgx_encl_shrink() and sgx_encl_add_page(), as I
> > pointed above? In this way there will be functional change in this patch, and
> > we need to explicitly explain  why leaving others out is OK in commit message.
> > 
> > To me I prefer the former.
> 
> The original purpose of this patch was exactly to remove EREMOVE
> sgx_free_epc_page() and call it explicitly where it is required. That's
> why I introduced sgx_reset_epc_page(). So the latter was actually the goal
> of this patch at least when I did it. Now this is something completely
> different.
> 
> So, I don't consider myself author of this patch in any possible way,
> because this is not what I intended.
> 
> To move forward, for the next patch set version, you should change the
> author field as yourself, and remove all my tags, and I will review it.
> So you can work out this with former approach if you wish.
> 
> I.e. my ack/nak/etc. apply to this patch because it's not my code.

OK. Thanks.

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