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

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Message-Id: <D4ENT15WZFRI.HA0FFVBWVISI@kernel.org>
Date: Tue, 24 Sep 2024 19:36:23 +0300
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "James Bottomley" <James.Bottomley@...senPartnership.com>,
 <linux-integrity@...r.kernel.org>
Cc: <roberto.sassu@...wei.com>, <mapengyu@...il.com>, "Mimi Zohar"
 <zohar@...ux.ibm.com>, "David Howells" <dhowells@...hat.com>, "Paul Moore"
 <paul@...l-moore.com>, "James Morris" <jmorris@...ei.org>, "Serge E.
 Hallyn" <serge@...lyn.com>, "Peter Huewe" <peterhuewe@....de>, "Jason
 Gunthorpe" <jgg@...pe.ca>, <keyrings@...r.kernel.org>,
 <linux-security-module@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 0/5] Lazy flush for the auth session

On Tue Sep 24, 2024 at 7:33 PM EEST, James Bottomley wrote:
> On Tue, 2024-09-24 at 19:29 +0300, Jarkko Sakkinen wrote:
> > On Tue Sep 24, 2024 at 4:48 PM EEST, James Bottomley wrote:
> [...]
> > > 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?
>
> Yes, because the null key is only used in session generation which is
> now lazy, it adds or subtracts nothing from the timings.  When you're
> forced to flush the session, the null key goes too, so you again have
> to restore it from the context.  When you can keep the session you
> don't need the null key because you're not regenerating it.

Yeah, OK, then we're in sync with this. It's evolutionary cruft.

Just had to check that the logic matches how I projected your earlier
comment because these are sensitive changes.

BR, Jarkko