Mr Tariq Kurd <tariq.kurd@...>
The threat model Tariq brought up was about detecting a glitch attack but the glitch can also happen when setting
It's not just for glitch protection, but also for security to control access permissions.
The software is loaded from the boot ROM and the boot ROM does not contain software to unlock the PMP entries so we are not (obviously) susceptible to a code-reuse ROP attack to reprogram the PMP, because the software to do so doesn't exist in the existing code.
If we need to lock regions to prevent such things from other software later then we need the flexibility to be able to lock the regions individually on a case by case basis.
Put simply we need orthogonal controls for locking and M-mode only access.
There are multiple use cases for the PMP, some we know about but many we don't and others won't have been thought of yet - so the proposal need to be flexible. Nick has a use case in mind which is perfectly valid with MML but unfortunately it doesn't cover our use case, and it also causes problems for Nvidia as Joe told us during the meeting on Tuesday.
Given that Andrew Waterman has dropped his objection to using an extra bit from the PMPCFG field we should allocate one.
So how about PMPCFG = M, which can be read-only-zero to exactly match the current PMP spec.
If MML=0, M=0 for any PMP entry, then the entry operates exactly as the current PMP spec.
If MML=0, M=1 for any PMP entry, then the RWX permissions are applied to all modes equally (see the attachment)
If MML=1, M=? for any PMP entry, then the RWX bits operate as they do for the MML proposal, except that the truth table has M,R,W,X as inputs instead of L,R,W,X (see the attachment)
For example, in the MML proposal:
- LRWX=1001 means "M-mode locked execute only region" - and it's locked
In my modified proposal:
- MRWX=1001 means "M-mode execute only region", and the L bit states whether it is locked.
I've attached the truth table in a word document - it's not a full proposal, just showing how to program the permissions.
I think this is a good simplification also for when MML=0 as it gives more flexibility than the current proposal.
So in summary this updated proposal gives us
1. M mode has all permissions and S/U modes have restricted permissions (unlocked) - M=0, MML=0, L=0
2. M mode and S/U modes all have the same permissions (locked) - M=0, MML=0, L=1
3. M mode and S/U modes all have the same permissions (unlocked) - M=1, MML=0, L=0
4. S/U mode permissions with no M-mode access (unlocked) - M=0, MML=1, L=0
5. S/U mode permissions with no M-mode access (locked) - M=0, MML=1, L=1
6. Shared regions (unlocked) - M=0, MML=1, L=0
7. Shared regions (locked) - M=0, MML=1, L=1
The existing standard gives us 1 and 2, the other modes are all new
From: tech-tee@... [mailto:tech-tee@...] On Behalf Of Nick Kossifidis
Sent: 14 February 2020 02:40
To: John Hauser <jh.riscv@...>
Cc: Nick Kossifidis <mick@...>; tech-tee@...; tech-privileged@...
Subject: Re: [RISC-V] [tech-tee] [RISC-V] [tech-privileged] comments on PMP enhancements
Στις 2020-02-13 22:30, John Hauser έγραψε:
Nick Kossifidis wrote:S/U mode doesn't have access to PMP registers so it's not possible to distinguish between an access fault e.g. due to a bug on an application / driver, from an access fault due to M mode trying to access one of the S/U-mode-only regions, or from an application / driver trying to access an M-mode-only region. It may be possible to recover from such a bug e.g. by restarting the application but handling / recovering from such a security violation is a different thing and usually involves different reporting and possibly running the system in a kind of "safe mode" with certain features disabled.The new mechanism (when MML is set) introduces a barrier between S/UI'm sorry to say, providing information to a debugger is not usually
So the idea is to have MXR also work for S/U-mode-only PMP regions ? I see how that would help on a system without an MMU, it'll also be more consistent this way, but on a system with MMU I don't see why it makes sense to use PMP to mark a region as execute-only. It makes more sense to mark the region as R/W/X for S/U mode and leave it to the OS running on S mode to add further restrictions through the MMU, in which case MXR will work as-is.mstatus.MXR is not related to PMP, it's related to virtual memoryThe case that needs to be dealt with is an S/U-mode-only region that
I agree with this approach as long as we make sure that MXR can only be set to 1 when running on M-mode, on the current spec it's allowed to be set regardless of privilege mode as with MPRV. I remember there was a discussion regarding MPRV to mandate it can only be set when on M-mode but I just checked the latest draft and it hasn't changed. Leaving MPRV set will most probably result the sw to crash, but if MXR is left set outside M-mode we basically remove the protection on any execute-only memory region, especially if this also works for PMP regions.
I just want to be sure that we are after specific issues and -in case of security controls- have specific threat models in mind. Coming up with solutions without having discussed the problem first and proposing security controls without a threat model won't work.Have in mind that this proposal isI believe we need to widen the scope of this proposal to cover other
Locking down mtvec may also be important, but since it doesn't involveStill MML can always be set after decompression/relocation/loading modules, I don't see why it's an issue and I don't see what's the added security gain by allowing temporary M-mode-only regions (removable) during boot or in general, where anything running there can remove them in a few instructions. The threat model Tariq brought up was about detecting a glitch attack but the glitch can also happen when setting a rule in the first place, I don't see how this is the proper approach, PMP is not there as an anti-tampering mechanism. If we want this as a debug feature I'm ok with it but it must be treated as such and not as a security improvement. Same goes for the ability to let M-mode still execute any region not covered by PMP, or being able to register new executable regions for "greater flexibility". We first need to decide if there are valid use cases / threat models that need to be addressed and then talk about possible modifications to the proposal.