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

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Date:   Sun, 15 Dec 2019 07:22:33 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        zohar@...ux.ibm.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 Fri, 2019-12-13 at 09:31 -0800, Lakshmi Ramasubramanian wrote:
> On 12/13/19 9:25 AM, James Bottomley wrote:
> 
> Hi James,
> 
> > 
> > There's no locking around the ima_process_keys flag.  If you get
> > two policy updates in quick succession can't this flag change as
> > you're processing the second update meaning you lose it because the
> > flag was false when you decided to build it for the queue but
> > becomes true before you check above whether you need to queue it?
> > 
> > Note you don't need locking to fix this, you just need to ensure
> > that you use the same copy of the flag value for both tests.
> > 
> > James
> > 
> 
> Same flag (ima_process_keys) is used for making the queuing decision.
> 
> Taking a lock to access ima_process_keys is required only if the flag
> is false. That is handled in ima_queue_key() and
> ima_process_queued_keys() functions.
> 
> Queued keys are processed when the first policy update occurs. 
> Subsequently, the keys are processed immediately (not queued).
> 
> Could you please review those functions in this patch and let me know
> if you see a problem?

This is the problem:

if (!flag)
    pre()
.
.
.
if (!flag)
    post()

And your pre and post function either have to both run or neither must.
 However, the flag is set asynchronously, so if it gets set while
another thread is running through the above code, it can change after
pre is run but before post is.

James