linux-kernel - Re: [PATCH v4 2/2] IMA: Call workqueue functions to measure queued keys

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Date:   Mon, 16 Dec 2019 08:05:15 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        linux-integrity@...r.kernel.org
Cc:     eric.snowberg@...cle.com, dhowells@...hat.com,
        mathew.j.martineau@...ux.intel.com, matthewgarrett@...gle.com,
        sashal@...nel.org, jamorris@...ux.microsoft.com,
        linux-kernel@...r.kernel.org, keyrings@...r.kernel.org
Subject: Re: [PATCH v4 2/2] IMA: Call workqueue functions to measure queued
 keys

On Mon, 2019-12-16 at 15:53 +0900, James Bottomley wrote:
> That doesn't matter ... the question is, is the input assumption that
> both pre/post have to be called or neither must correct?  If so, the
> code is wrong, if not, explain why.

Thanks, James, for looking at the locking.

"ima_process_keys" is set once.  Once it is set, the keys are measured
immediately.  For performance to avoid taking the mutex, both the
reader and writer check "ima_process_keys" twice, once without taking
the lock and, again, after taking the lock.  Based on the second test,
the reader queues the "key" or not.  Refer to ima_queue_key().

The latest patch version sets "ima_process_keys" after taking the
lock.  With this change, the comment in ima_process_queued_keys() is
now correct.  We're now guaranteed to process the queued "keys" just
once and not drop any "key" measurements.

I hope this answers your question.

Mimi