linux-kernel - Re: [PATCH] ima: fix deadlock within RCU list of ima

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Date:   Wed, 25 Aug 2021 12:03:50 +0000
From:   THOBY Simon <Simon.THOBY@...eris.fr>
To:     liqiong <liqiong@...china.com>, Mimi Zohar <zohar@...ux.ibm.com>
CC:     "dmitry.kasatkin@...il.com" <dmitry.kasatkin@...il.com>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "serge@...lyn.com" <serge@...lyn.com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ima: fix deadlock within RCU list of ima_rules

Hi Liqiong,

On 8/25/21 1:45 PM, liqiong wrote:
> Hi Mimi,
> 
> This copy may be better.
> 
> 
> subject: ima: fix deadlock when iterating over the init "ima_rules" list.
> 
> 

As Mimi said, consider adding an introducing paragraph, like:
'The current IMA ruleset is identified by the variable "ima_rules",
and the pointer starts pointing at the list "ima_default_rules". When
loading a custom policy for the first time, the variable is
updated to point to the list "ima_policy_rules" instead. That update
isn't RCU-safe, and deadlocks are possible.'

> 
> When traversing back to head, the init "ima_rules" list can't exit
> iterating if "ima_rules" has been updated to "ima_policy_rules".
> It causes soft lockup and RCU stalls. So we can introduce a duplicate
> of "ima_rules" for each "ima_rules" list loop.

Per the process (see 'Documentation/process/submitting-patches.rst'),
please prefer an imperative style (written in another paragraph):
'Introduce a temporary value for "ima_rules" when iterating over the ruleset.'


Thanks,
Simon