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

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Message-ID: <3ef6906b-ccf1-0ab8-180b-b71568e22443@viveris.fr>
Date:   Thu, 26 Aug 2021 09:01:04 +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 8/26/21 10:15 AM, liqiong wrote:
> Hi Simon,
> 
> Thanks for your help, your advice is clear, can i just use it,
> how about this:
> 
> 
> The current IMA ruleset is identified by the variable "ima_rules",
> and the pointer starts pointing at the list "ima_default_rules".

After reading it again, maybe
"The current IMA ruleset is identified by the variable "ima_rules",
that defaults to "&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.

I think what Mimi was trying to say is that you could add the high-level
overview above, but keep the details. Sorry if I wasn't clearer in my
earlier messages.

Consider re-adding your previous paragraph
"""
As a consequence, when traversing the ruleset, some functions like ima_match_policy()
may loop indefinitely over "ima_default_rules" as list_for_each_entry_rcu() doesn't
terminate (after the update, "ima_rules" no longer points to the list head, so the
loop condition stays always true), causing RCU stalls.
"""
(note: I tweaked it above, feel free to fix it)
> 
> Introduce a temporary value for "ima_rules" when iterating over
> the ruleset to avoid the deadlocks.

... while keeping this a separate paragraph.

> 
> 
> 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