Re: [seL4] Change Access Right to Memory Pages

Actually, we want to try various methods to provide a consistent snapshot of memory. One method is to perform a bulk reduction in access rights of a lot of (mapped) memory pages. Then, compute a hash on those pages. After that, we increase the access rights back to the original (or R/W). In the real implementation, we would like to handle when a legitimate page fault happens (e.g. when another authorized thread tries to write into that page while the page is being locked). Then, we have to increase access rights and do copy-on-write-like operations on that page. Oak On Tue, May 2, 2017 at 9:22 PM, wrote:

Just put up a straw man to potentially correct my understanding.

I interpret this conversation as desiring a primitive to perform a bulk reduction in access rights of range page table entries of an address space – something that seems relatively straight forward – ignoring atomicity/WCET issues.

What is not as straightforward is increasing access rights as appropriate authority needs to be presented somehow. If R/O -> R/W is page fault driven, e.g. for copy-on-write-like implementations, then that is relatively simple.

- Kevin

*From:* Devel [mailto:devel-bounces@sel4.systems] *On Behalf Of *Norrathep Rattanavipanon *Sent:* Tuesday, 2 May 2017 4:07 PM *To:* Heiser, Gernot (Data61, Kensington NSW) < Gernot.Heiser@data61.csiro.au> *Cc:* devel@sel4.systems *Subject:* Re: [seL4] Change Access Right to Memory Pages

Yes, you are absolutely right about that. I think it would be good to include the map time (even though called it once) for completeness :).

On Mon, May 1, 2017 at 10:36 PM, wrote:

Hi Oak,

My understanding is that in your use case, the map is a one-off and therefore not performance-critical, while the re-map needs to be done on each snapshot and therefore is performance-critical.

I hope the student gets to evaluate alternative implementation so we can see how much benefit bulk operations would provide.

Gernot

On 2 May 2017, at 15:32 , Norrathep Rattanavipanon wrote:

Hi Adrain and Gernot,

Thank you for the clarification. Yes, sorry, forgot to mention that we are using ARM i.MX6 Sabre Lite and forgot to mention some backgrounds.

We are interested in benchmarking (1) the time to lock large memory regions (switching from R/W to R/O) and (2) the time to map large memory regions as Gernot mentioned its for providing a consistent snapshot of user memory.

So just to make sure the current implementation for that is the optimal remap (modify paging structure, clean cache, invalidate tlb), correct?

@Gernot, right, per our discussion, we only discussed out a bulk mapping operation (mapping from other processes into the root process) but I was not sure about locking (which is remapping in this case). Thank you for confirming.

Best,

Oak

On Mon, May 1, 2017 at 10:18 PM, wrote:

On 2 May 2017, at 3:21 , Norrathep Rattanavipanon wrote:

2) If my understanding in 1) is correct, the remap function only takes 1 cap at a time.

So if I want to modify access rights of many pages, do I have to call the remap function multiple times?

Or is there a more efficient way to do so?

Hi Oak,

As we discussed in person, this is a known issue of our API, and I have presently a student doing some evaluations. I have your use case in mind when talking to the student – for everyone else’s benefit, this is about a consistent shapshot of user memory for doing remote attestation.

I would be interested in other use cases for a bulk re-mapping operation. I suspect that there are use cases where this might be a bottleneck, but it would be good to identify specific cases of enough significance to consider an API change. Certainly there won’t be a change if we don’t have convincing use cases.

Gernot

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Norrathep (Oak) Rattanavipanon

M.S. in Computer Science

University of California - Irvine

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Norrathep (Oak) Rattanavipanon

M.S. in Computer Science

University of California - Irvine

-- Norrathep (Oak) Rattanavipanon M.S. in Computer Science University of California - Irvine