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Subject: Re: [PATCH] ima: fix deadlock within "ima_match_policy" function.
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     liqiong <liqiong@nfschina.com>,
        THOBY Simon <Simon.THOBY@viveris.fr>
Cc:     "dmitry.kasatkin@gmail.com" <dmitry.kasatkin@gmail.com>,
        "jmorris@namei.org" <jmorris@namei.org>,
        "serge@hallyn.com" <serge@hallyn.com>,
        "linux-integrity@vger.kernel.org" <linux-integrity@vger.kernel.org>,
        "linux-security-module@vger.kernel.org" 
        <linux-security-module@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Date:   Tue, 24 Aug 2021 08:38:43 -0400
In-Reply-To: <3ba4da9d-fa7b-c486-0c48-67cee4d5de6d@nfschina.com>
References: <20210819101529.28001-1-liqiong@nfschina.com>
         <20210824085747.23604-1-liqiong@nfschina.com>
         <e720e88e-ebfa-56df-6048-f2da0b8fa2a0@viveris.fr>
         <3ba4da9d-fa7b-c486-0c48-67cee4d5de6d@nfschina.com>
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On Tue, 2021-08-24 at 20:09 +0800, liqiong wrote:
> Hi Simon :
> 
> ima: fix deadlock within RCU list of ima_rules.
> 

Before the following paragraph, an introductory sentence is needed. 
Try adding a sentence to the affect that "ima_rules" initially points
to the "ima_default_rules", but after loading a custom policy points to
the "ima_policy_rules".   Then describe the bug at a high level,
something like - transitioning to the "ima_policy_rules" isn't being
done safely.

Followed by the details.

> ima_match_policy() is looping on the policy ruleset while
> ima_update_policy() updates the variable "ima_rules". This can
> lead to a situation where ima_match_policy() can't exit the
> 'list_for_each_entry_rcu' loop, causing RCU stalls
> ("rcu_sched detected stall on CPU ...").
> 
> This problem can happen in practice: updating the IMA policy
> in the boot process while systemd-services are being checked.
> 
> In addition to ima_match_policy(), other function with 
> "list_for_each_entry_rcu" should happen too. Fix locking by 
> introducing a duplicate of "ima_rules" for each 
> "list_for_each_entry_rcu".
> 
> 
> How about this commit message ?
> 
> I have tested this patch in lab, we can reproduced this error case, 
> have done reboot test many times. This patch should work. 

The above comment doesn't belong in the commit message, but is a
message to the reviewers/maintainers and goes after the patch
descriptions three dashes line.

thanks,

Mimi