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

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Message-ID: <caa4955e-04d9-d3f8-cccb-5f78bd554c55@viveris.fr>
Date:   Fri, 27 Aug 2021 07:30:55 +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,

a few nits but nothing significant. This is getting in good shape!

On 8/27/21 8:41 AM, liqiong wrote:
> Hi Simon,
> 
> Thanks for you help, i may got it, here is the new commit message:
> 
> 
> The current IMA ruleset is identified by the variable "ima_rules"
> that default to "&ima_default_rules". When loading a custom policy
> for the first time, the variable is updated to "&ima_policy_rules"
> instead. That update isn't RCU-safe, and deadlocks are possible.
> Because some functions like ima_match_policy() may loop indefinitely

s/Because/Indeed,/ (in english, sentences with a subordinating conjunction
like 'because' are usually written in two parts, and a dependent clause
standing by itself is rarely used except for stylistic effect)

> over traversing the "ima_default_rules" as list_for_each_entry_rcu().

s/over traversing the "ima_default_rules" as list_for_each_entry_rcu()/when traversing "ima_default_rules" with list_for_each_entry_rcu()./

> 
> When iterating over the default ruleset back to head, value of
> "&entry->list" is "&ima_default_rules", and "ima_rules" may have been

s/value of "&entry->list" is "&ima_default_rules"/if the list head is "ima_default_rules"/
s/may have been/have been/

> updated to "&ima_policy_rules", the loop condition (&entry->list != ima_rules)
> stay alway true, traversing doesn't terminate, cause soft lockup and

Don't forget the 's' in "stays" (or "remains")
Ditto for "always"
s/traversing doesn't/traversing won't/
Also: s/cause/causing a/

> RCU stalls.
> 
> Introduce a temporary value for "ima_rules" when iterating over
> the ruleset to avoid the deadlocks.
> 
> 
> Signed-off-by: liqiong <liqiong@...china.com>
> ---
>  security/integrity/ima/ima_policy.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index fd5d46e511f1..e92b197bfd3c 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> 
> 
> Thanks
> 
> liqiong
> 

Thanks,
Simon