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@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. > > 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
