Messages in this thread | | | Date | Tue, 17 Jan 2017 18:29:23 +0200 | From | Jarkko Sakkinen <> | Subject | Re: [PATCH] tpm: add session handles to the save and restore of the tpm2 space manager |
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On Tue, Jan 17, 2017 at 06:18:12AM -0800, James Bottomley wrote: > On Tue, 2017-01-17 at 09:23 +0200, Jarkko Sakkinen wrote: > > On Mon, Jan 16, 2017 at 03:18:45PM -0800, James Bottomley wrote: > > > On Mon, 2017-01-16 at 12:04 +0200, Jarkko Sakkinen wrote: > > > > On Fri, Jan 13, 2017 at 11:24:13AM -0800, James Bottomley wrote: > > > > > Session handles are slightly more difficult to manage because > > > > > any > > > > > TPM > > > > > only has a finite number of allowed handles, even if the > > > > > session > > > > > has > > > > > been saved; so when you context save a session, you must not > > > > > flush > > > > > it > > > > > because that would destroy the ability to context load it (you > > > > > only > > > > > flush sessions when you're done with them) and the tpm won't re > > > > > -use > > > > > the handle. Additionally, sessions can be flushed as part of > > > > > the > > > > > successful execution of a command if the continueSession > > > > > attribute > > > > > is > > > > > clear, so we have to mark any session we find in the command > > > > > (using > > > > > TPM2_HT_TAG_FOR_FLUSH) so it can be cleared from the space if > > > > > the > > > > > command successfully executes. > > > > > > > > > > Finally, a session may also be cleared by flushing it, so we > > > > > have > > > > > to > > > > > emulate the TPM2_FlushContext command to see if a session is > > > > > being > > > > > flushed and manually clear it from the space. > > > > > > > > > > We also fully flush all sessions on device close. > > > > > > > > Some big overview comments without going deeply into details. I > > > > will > > > > use more time for this once the > > > > > > > > Please do not use handle allocation code for sessions. This > > > > commit > > > > makes the implementation a mess. Just use the phandle directly > > > > and > > > > have array of session phandles for each space. > > > > > > > > I would also almost require to have at minimum two patches: one > > > > that > > > > implements purely isolation and another that implements swapping. > > > > > > > > It might be for example that I want to land TPM spaces with > > > > session > > > > isolation to one release and swapping to n+1 because my hunch > > > > tells > > > > me that it is better to bake the swapping part for a while. > > > > > > > > One more thing. Maybe commit messages should speak uniformally > > > > about > > > > TPM spaces? They are a tool to implement resource manager, not a > > > > resource manager. > > > > > > Yes, so Ken also had a reply to this which the Mailing List seems > > > to > > > have eaten: > > > > > > > This looks like session handles are virtualized. I believe that > > > > this > > > > will break the HMAC for commands (e.g. TPM2_PolicySecret) that > > > > have > > > > a session handle in the handle area. The session's handle is its > > > > "Name" and is included in the cpHash (command parameter hash) > > > > data > > > > that is HMACed. > > > > > > Basically this means that the advice to virtualize session handles > > > in > > > the TCG RM document is wrong and we have to use physical handles. > > > I'll > > > redo the implementation for this ... and now, since we'll have > > > nothing > > > to use as an index, it probably does make sense to have sessions in > > > a > > > separate array. I can also separate isolation from context > > > switching > > > ... although I really think this is less optimal: my TPM only > > > allows > > > three active context handles, so if we don't context switch them > > > identially to transient object (which it also only allows three of) > > > I'm > > > going to run out. I actually redid my openssl_tpm_engine patches > > > so > > > they use session handles for parameter encryption and HMAC based > > > authority, so this may end up biting me soon ... > > > > > > James > > > > This might be obvious to you but saying this just in case: everytime > > a session handle is created, you must traverse through struct > > tpm_space instances and check if they have that physical handle and > > remove it so that they don't leak. > > Actually, the code pre-emptively manages handles, so it ensures that we > actively collect them (in the flush emulation and the command attribute > processing). I've got an unpublished patch that actually does global > session management, but it's part of the resource exhaustion stuff > which I'm still testing. > > Note that when we save contexts, there's no need even to check. If a > handle gets re-used, the old context won't load.
Yes, I understand. In that way swapping simplifies things like it does with transient objects.
> > I would probably just create a linked list of struct tpm_space > > instances. Radix tree does not make much sense with the amount of > > sessions you might except to have on a system simultaneously. > > Global array. The tpm has a small limit for total number of sessions > TPM_PT_SESSIONS_MAX (it's about 64 in every TPM I've seen).
[x]
> > Anyway, this kind of lazy approach where you clean up as new stuff > > gets created is probably also the most straight forward... > > I think we need to manage handles proactively. The problems come when > we have more saved contexts than TPM_PT_SESSIONS_MAX, so we need to > aggressively flush them.
I leave it up to you decide how you want to do it. In some ways swapping does simplify things so it's not yet obvious whether a kind of breakdown where isolation would be done as an iterative step would make sense in the first place.
Forget what I adviced and I'll look forward for a new patch :-)
/Jarkko
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