Re: [PATCH v5 0/5] Lazy flush for the auth session

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Tue Sep 24, 2024 at 4:48 PM EEST, James Bottomley wrote:
> On Sun, 2024-09-22 at 20:51 +0300, Jarkko Sakkinen wrote:
> > On Sat Sep 21, 2024 at 3:08 PM EEST, Jarkko Sakkinen wrote:
> > > This patch set aims to fix:
> > > https://bugzilla.kernel.org/show_bug.cgi?id=219229.
> > > 
> > > The baseline for the series is the v6.11 tag.
> > > 
> > > v4:
> > > https://lore.kernel.org/linux-integrity/20240918203559.192605-1-jarkko@kernel.org/
> > > v3:
> > > https://lore.kernel.org/linux-integrity/20240917154444.702370-1-jarkko@kernel.org/
> > > v2:
> > > https://lore.kernel.org/linux-integrity/20240916110714.1396407-1-jarkko@kernel.org/
> > > v1:
> > > https://lore.kernel.org/linux-integrity/20240915180448.2030115-1-jarkko@kernel.org/
> > > 
> > > Jarkko Sakkinen (5):
> > >   tpm: Return on tpm2_create_null_primary() failure
> > >   tpm: Implement tpm2_load_null() rollback
> > >   tpm: flush the null key only when /dev/tpm0 is accessed
> > >   tpm: Allocate chip->auth in tpm2_start_auth_session()
> > >   tpm: flush the auth session only when /dev/tpm0 is open
> > > 
> > >  drivers/char/tpm/tpm-chip.c       |  14 ++++
> > >  drivers/char/tpm/tpm-dev-common.c |   8 +++
> > >  drivers/char/tpm/tpm-interface.c  |  10 ++-
> > >  drivers/char/tpm/tpm2-cmd.c       |   3 +
> > >  drivers/char/tpm/tpm2-sessions.c  | 109 ++++++++++++++++++--------
> > > ----
> > >  include/linux/tpm.h               |   2 +
> > >  6 files changed, 102 insertions(+), 44 deletions(-)
> > 
> > 
> > Roberto, James, speaking of digest cache. This patch set has no aim
> > to fix those issues but I do believe that it should improve also that
> > feature.
> > 
> > If I don't get soon patch reviews for the patch set, I'll pick the
> > 2nd best option: disable bus encryption on all architectures
> > including x86 and ARM64 (being by default on).
> > 
> > It's a force majeure situation. I know this would sort out the issue
> > but I really cannot send these as a pull request with zero reviewe-
> > by's.
> > 
> > I expect this to be closed by tomorrow.
>
> Hey come on, you knew I was running plumbers last week so I had all the
> lead up and teardown stuff to do as well.  I'm only just digging
> through accumulated email.

Fair enough, I actually do not want to disable the feature. That
was my main concern here. Now if we get this fixed we might be
able to revisit earlier decisions on defconfig and widen the
support eventually, not shrink it.


>
> Patches 1-2 are fully irrelevant to the bug, so I ignored them on the
> grounds that improvement to the error flow could be done through the
> normal patch process

Hmm.. I'll revisit this for v6. Not sure what to say on this yet
because I need to address the other remarks and based on that
reflect. So might drop or keep them but not 100% sure yet.


> Patch 3 is completely unnecessary: the null key is only used to salt
> the session and is not required to be resident while the session is
> used (so can be flushed after session creation) therefore keeping it
> around serves no purpose once the session is created and simply
> clutters up the TPM volatile handle slots. (I don't know of a case
> where we use all the slots in a kernel operation, but since we don't
> need it lets not find out when we get one).  So I advise dropping patch
> 3.

Let's go this through just to check I'm understanding.

Holding null key had radical effect on boot time: it cut it down by
5 secons down to 15 seconds:

https://lore.kernel.org/linux-integrity/CALSz7m1WG7fZ9UuO0URgCZEDG7r_wB4Ev_4mOHJThH_d1Ed1nw@mail.gmail.com/

Then in subsequent version I implemented lazy auth session and boot
time went down to 9.7 seconds.

So is the point you're trying to make that since auth session is 
already held as long as we can and they flushed in synchronous
point too, I can just as well drop patch 3?

I think I reach your point but just want to check that I do it
for the matching reasons. It is evolutionary cruft in the patch
set :-)

BR, Jarkko