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@xxxxxxxxxx> 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@xxxxxxxxxx> > > >>>>>> 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. /Jarkko
