linux-kernel - Re: [PATCH 0/4] tpm: lazy flush for the session null key

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Message-ID: <CALSz7m1qd-7uLKbQ9Dz36GvzompQ7gg7Vfe2uj3YRj0jHONDPg@mail.gmail.com>
Date: Mon, 16 Sep 2024 13:53:02 +0800
From: Pengyu Ma <mapengyu@...il.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: linux-integrity@...r.kernel.org, James.Bottomley@...senpartnership.com, 
	roberto.sassu@...wei.com, Peter Huewe <peterhuewe@....de>, 
	Jason Gunthorpe <jgg@...pe.ca>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] tpm: lazy flush for the session null key

On Mon, Sep 16, 2024 at 1:16 PM Jarkko Sakkinen <jarkko@...nel.org> wrote:
>
> On Mon Sep 16, 2024 at 5:33 AM EEST, Pengyu Ma wrote:
> > After applied your patches, the boot time is ~15 seconds.
> > Less than 20 sec, but still much more than 7 sec when disabling HMAC.
>
> Great, and thank you for testing this. I did expect it to fully address
> the issue but it is on the direct path. It took me few days to get my
> testing environment right before moving forward [1], mainly to get
> bpftrace included, thus the delay.
>
> Do you mind if I add tested-by for the for this one?
>

Yes, please feel free to add it.
And thanks for the effort and details.

BR,
Pengyu.


> Before the patch set the in-kernel TPM sequences were along the lines
> of:
>
> 1. Load the null key.
> 2. Load the auth session.
> 3. Do stuff with overhead from encryption.
> 4. Save the session.
> 5. Save the null key.
>
> With the changes:
>
> 1. Load the session.
> 2. Do stuff with overhead from encryption.
> 3. Save the session.
>
> Each swapped session gets an increasing count. If the count grows over
> treshold measured by the difference of the count in the latest loaded
> session and the session currently being saved, then TPM throws out
> a context gap error. It has a limited resolution for this.
>
> As long as /dev/tpm0 is not opened by any process, there is only one
> session open (or at least fixed pre-determined number moving forward).
> This means that context gap error cannot occur, as the only session
> saved is the auth session.
>
> I'll implement a patch on top of this, which does exactly this: track
> the number of open /dev/tpm{rm0}. Only when the device is open, the
> auth session is flushed.
>
> With this change the sequence reduces to:
>
> 1. Do stuff with overhead from encryption.
>
> Since the results are promising (thanks to you), I create a new version
> of this patch set with this additional fix. There's no chance to reach
> the same exact boot-up time as without encryption but I think we might
> be able to reach a reasonable cost.
>
> [1] https://codeberg.org/jarkko/linux-tpmdd-test
>
> BR, Jarkko