Message-ID: <YeH6d5KEnT6xfpyP@iki.fi>
Date:   Sat, 15 Jan 2022 00:34:31 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Nathaniel McCallum <nathaniel@...fian.com>,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, dave.hansen@...ux.intel.com,
        tglx@...utronix.de, bp@...en8.de, mingo@...hat.com,
        linux-sgx@...r.kernel.org, x86@...nel.org, seanjc@...gle.com,
        kai.huang@...el.com, cathy.zhang@...el.com, cedric.xing@...el.com,
        haitao.huang@...el.com, mark.shanahan@...el.com, hpa@...or.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/25] x86/sgx: Introduce runtime protection bits

On Sat, Jan 15, 2022 at 12:23:46AM +0200, Jarkko Sakkinen wrote:
> On Sat, Jan 15, 2022 at 12:17:06AM +0200, Jarkko Sakkinen wrote:
> > On Fri, Jan 14, 2022 at 11:53:22PM +0200, Jarkko Sakkinen wrote:
> > > On Thu, Jan 13, 2022 at 01:42:50PM -0800, Reinette Chatre wrote:
> > > > Hi Jarkko and Nathaniel,
> > > > 
> > > > On 1/13/2022 12:09 PM, Nathaniel McCallum wrote:
> > > > > On Wed, Jan 12, 2022 at 6:56 PM Jarkko Sakkinen <jarkko@...nel.org> wrote:
> > > > >>
> > > > >> On Thu, Jan 13, 2022 at 01:50:13AM +0200, Jarkko Sakkinen wrote:
> > > > >>> On Tue, Jan 11, 2022 at 09:13:27AM -0800, Reinette Chatre wrote:
> > > > >>>> Hi Jarkko,
> > > > >>>>
> > > > >>>> On 1/10/2022 5:53 PM, Jarkko Sakkinen wrote:
> > > > >>>>> On Mon, Jan 10, 2022 at 04:05:21PM -0600, Haitao Huang wrote:
> > > > >>>>>> On Sat, 08 Jan 2022 10:22:30 -0600, Jarkko Sakkinen <jarkko@...nel.org>
> > > > >>>>>> wrote:
> > > > >>>>>>
> > > > >>>>>>> On Sat, Jan 08, 2022 at 05:51:46PM +0200, Jarkko Sakkinen wrote:
> > > > >>>>>>>> On Sat, Jan 08, 2022 at 05:45:44PM +0200, Jarkko Sakkinen wrote:
> > > > >>>>>>>>> On Fri, Jan 07, 2022 at 10:14:29AM -0600, Haitao Huang wrote:
> > > > >>>>>>>>>>>>> OK, so the question is: do we need both or would a
> > > > >>>>>>>> mechanism just
> > > > >>>>>>>>>>>> to extend
> > > > >>>>>>>>>>>>> permissions be sufficient?
> > > > >>>>>>>>>>>>
> > > > >>>>>>>>>>>> I do believe that we need both in order to support pages
> > > > >>>>>>>> having only
> > > > >>>>>>>>>>>> the permissions required to support their intended use
> > > > >>>>>>>> during the
> > > > >>>>>>>>>>>> time the
> > > > >>>>>>>>>>>> particular access is required. While technically it is
> > > > >>>>>>>> possible to grant
> > > > >>>>>>>>>>>> pages all permissions they may need during their lifetime it
> > > > >>>>>>>> is safer to
> > > > >>>>>>>>>>>> remove permissions when no longer required.
> > > > >>>>>>>>>>>
> > > > >>>>>>>>>>> So if we imagine a run-time: how EMODPR would be useful, and
> > > > >>>>>>>> how using it
> > > > >>>>>>>>>>> would make things safer?
> > > > >>>>>>>>>>>
> > > > >>>>>>>>>> In scenarios of JIT compilers, once code is generated into RW pages,
> > > > >>>>>>>>>> modifying both PTE and EPCM permissions to RX would be a good
> > > > >>>>>>>> defensive
> > > > >>>>>>>>>> measure. In that case, EMODPR is useful.
> > > > >>>>>>>>>
> > > > >>>>>>>>> What is the exact threat we are talking about?
> > > > >>>>>>>>
> > > > >>>>>>>> To add: it should be *significantly* critical thread, given that not
> > > > >>>>>>>> supporting only EAUG would leave us only one complex call pattern with
> > > > >>>>>>>> EACCEPT involvement.
> > > > >>>>>>>>
> > > > >>>>>>>> I'd even go to suggest to leave EMODPR out of the patch set, and
> > > > >>>>>>>> introduce
> > > > >>>>>>>> it when there is PoC code for any of the existing run-time that
> > > > >>>>>>>> demonstrates the demand for it. Right now this way too speculative.
> > > > >>>>>>>>
> > > > >>>>>>>> Supporting EMODPE is IMHO by factors more critical.
> > > > >>>>>>>
> > > > >>>>>>> At least it does not protected against enclave code because an enclave
> > > > >>>>>>> can
> > > > >>>>>>> always choose not to EACCEPT any of the EMODPR requests. I'm not only
> > > > >>>>>>> confused here about the actual threat but also the potential adversary
> > > > >>>>>>> and
> > > > >>>>>>> target.
> > > > >>>>>>>
> > > > >>>>>> I'm not sure I follow your thoughts here. The sequence should be for enclave
> > > > >>>>>> to request  EMODPR in the first place through runtime to kernel, then to
> > > > >>>>>> verify with EACCEPT that the OS indeed has done EMODPR.
> > > > >>>>>> If enclave does not verify with EACCEPT, then its own code has
> > > > >>>>>> vulnerability. But this does not justify OS not providing the mechanism to
> > > > >>>>>> request EMODPR.
> > > > >>>>>
> > > > >>>>> The question is really simple: what is the threat scenario? In order to use
> > > > >>>>> the word "vulnerability", you would need one.
> > > > >>>>>
> > > > >>>>> Given the complexity of the whole dance with EMODPR it is mandatory to have
> > > > >>>>> one, in order to ack it to the mainline.
> > > > >>>>>
> > > > >>>>
> > > > >>>> Which complexity related to EMODPR are you concerned about? In a later message
> > > > >>>> you mention "This leaves only EAUG and EMODT requiring the EACCEPT handshake"
> > > > >>>> so it seems that you are perhaps concerned about the flow involving EACCEPT?
> > > > >>>> The OS does not require nor depend on EACCEPT being called as part of these flows
> > > > >>>> so a faulty or misbehaving user space omitting an EACCEPT call would not impact
> > > > >>>> these flows in the OS, but would of course impact the enclave.
> > > > >>>
> > > > >>> I'd say *any* complexity because I see no benefit of supporting it. E.g.
> > > > >>> EMODPR/EACCEPT/EMODPE sequence I mentioned to Haitao concerns me. How is
> > > > >>> EMODPR going to help with any sort of workload?
> > > > >>
> > > > >> I've even started think should we just always allow mmap()?
> > > > > 
> > > > > I suspect this may be the most ergonomic way forward. Instructions
> > > > > like EAUG/EMODPR/etc are really irrelevant implementation details to
> > > > > what the enclave wants, which is a memory mapping in the enclave. Why
> > > > > make the enclave runner do multiple context switches just to change
> > > > > the memory map of an enclave?
> > > > 
> > > > The enclave runner is not forced to make any changes to a memory mapping. To start,
> > > > this implementation supports and does not change the existing ABI where a new
> > > > memory mapping can only be created if its permissions are the same or weaker
> > > > than the EPCM permissions. After the memory mapping is created the EPCM permissions
> > > > can change (thanks to SGX2) and when they do there are no forced nor required
> > > > changes to the memory mapping - pages remain accessible where the memory mapping
> > > > and EPCM permissions agree. It is true that if an enclave chooses to relax permissions
> > > > to an enclave page (EMODPE) then the memory mapping may need to be changed as
> > > > should be expected to access a page with permissions that the memory mapping
> > > > did not previously allow.
> > > > 
> > > > Are you saying that the permissions of a new memory mapping should now be allowed
> > > > to exceed EPCM permissions and thus the enclave runner would not need to modify a
> > > > memory mapping when EPCM permissions are relaxed? As mentioned above this may be
> > > > considered a change in ABI but something we could support on SGX2 systems.
> > > > 
> > > > I would also like to highlight Haitao's earlier comment that a foundation of SGX is
> > > > that the OS is untrusted. The enclave owner does not trust the OS and needs EMODPR
> > > > and EMODPE to manage enclave page permissions.
> > > 
> > > Thanks, this was very informative response. I'll try to elaborate why
> > > EMODPR gives me headaches.
> > > 
> > > I'm having hard time to connect the dots between OS mistrust and
> > > restricting enclave by changing EPCM permissions. To make EMODPR actually
> > > legit, it needs really at least some sort of example of a scenario where
> > > mistrusted OS is the adversary and enclave is the attack target. Otherwise,
> > > we are just waving our hands.
> > > 
> > > Generally speaking a restriction is not a restriction if cannot be enforced. 
> > > 
> > > I see two non-EMODPR options: you could relax this,  *or* you could make it
> > > soft restriction by not doing EMODPR but instead just updating the internal
> > > xarray. The 2nd option would be fully backwards compatible with the
> > > existing invariant.
> > > 
> > > It's really hard to ACK or NAK EMODPR patch without knowing how EMODPE is
> > > or will be supported.
> > 
> > I think I *might* have a supporting scenario for EMODPR.
> > 
> > Enclave might want to accept EMODPR request because a bug in functionality
> > triggered with TCS entries might allow otherwise to rewrite enclave data,
> > i.e. provide a write primitive outside the enclave. With some other way to
> > exploit you could have a read primitive and thus have a full access to the
> > internal data of the enclave.
> 
> I.e. because of this it would be "for profit case" for the enclave not to
> cancel the effect of EMODPR by applying EMODPE because it can protect
> itself by doing that from malformed input data.
> 
> I get that the whole point is the OS mistrust but you really need to bring
> up the rationale to the specifics what you mean by it in the context of the
> kernel patch. Otherwise, anything would go by saying that we do this
> because OS mistrust.

My scenario is illegit because:

1. An attacker can choose not to do EMODPR and still take advantage of the
   exploit, and get the write primitive.
2. Enclave has very theoretical chances to counter-measure that because
   introspection is not possible, only the "mistrusted OS" has that
   capability, i.e. the attacker. ERDINFO is AFAIK ENCLS leaf.

/Jarkko

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