linux-kernel - Re: [PATCH] audit: catch possible NULL audit buffers

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Date:	Mon, 19 Nov 2012 14:26:59 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	Eric Paris <eparis@...hat.com>
Subject: Re: [PATCH] audit: catch possible NULL audit buffers

On Mon, Nov 19, 2012 at 2:14 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Mon, 19 Nov 2012 14:00:51 -0800
> Kees Cook <keescook@...omium.org> wrote:
>
>> It's possible for audit_log_start() to return NULL. Handle it in the
>> various callers.
>>
>> ...
>>
>> --- a/kernel/audit.c
>> +++ b/kernel/audit.c
>> @@ -272,6 +272,8 @@ static int audit_log_config_change(char *function_name, int new, int old,
>>       int rc = 0;
>>
>>       ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_CONFIG_CHANGE);
>> +     if (unlikely(!ab))
>> +             return rc;
>
> Returning success here looks suspicious.  audit_do_config_change() will
> fail to take its wtf-just-happened action (which
> audit_log_config_change() duplicates, btw).

It seemed like the only meaningful rc was from when
security_secid_to_secctx failed, which changes the audit report and
has external side-effects that seem specific to that condition. It
seemed that audit_log_start() failing was less of a problem than
security_secid_to_secctx failing, which seems supposed be all the
other code in the kernel that gives up if audit_log_start() fails.

> Meanwhile audit_receive_msg() is off living in a happy land where
> nothing ever goes wrong.

Right, this reinforced my perceptions that a audit_log_start() failure
wasn't meaningful to propagate to callers of
audit_log_config_change().

>
>>       audit_log_format(ab, "%s=%d old=%d auid=%u ses=%u", function_name, new,
>>                        old, from_kuid(&init_user_ns, loginuid), sessionid);
>>       if (sid) {
>> @@ -619,6 +621,8 @@ static int audit_log_common_recv_msg(struct audit_buffer **ab, u16 msg_type,
>>       }
>>
>>       *ab = audit_log_start(NULL, GFP_KERNEL, msg_type);
>> +     if (unlikely(!*ab))
>> +             return rc;
>
> Also looks fishy.

Without any callers checking the return value, it seemed useless to
propagate here too.

-Kees

-- 
Kees Cook
Chrome OS Security
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