linux-kernel - Re: [PATCH v2] security_syslog() should be called once only

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Message-ID: <556D61D4.9090304@virtuozzo.com>
Date:	Tue, 02 Jun 2015 10:57:08 +0300
From:	Vasily Averin <vvs@...tuozzo.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
	Josh Boyer <jwboyer@...hat.com>, Eric Paris <eparis@...hat.com>
Subject: Re: [PATCH v2] security_syslog() should be called once only

On 02.06.2015 00:23, Andrew Morton wrote:
> On Sat, 30 May 2015 16:51:34 +0300 Vasily Averin <vvs@...tuozzo.com> wrote:
> 
>> On 28.05.2015 02:43, Andrew Morton wrote:
>>> So we run security_syslog() for actions other than open() (of kmsg). 
>>> Why?
>> Could you please clarify this question?
>>
>> Linux kernel have reasonable default security policy and it's great.
>> And at the same time kernel allows to override default behaviour
>> and set custom security policy.
>> For example, to prohibit work on Saturday.
>> QA can use it for random failures generation.
>> Why not?
> 
> This change:
> 
> : --- a/kernel/printk/printk.c~security_syslog-should-be-called-once-only
> : +++ a/kernel/printk/printk.c
> : @@ -496,11 +496,11 @@ int check_syslog_permissions(int type, b
> :  	 * already done the capabilities checks at open time.
> :  	 */
> :  	if (from_file && type != SYSLOG_ACTION_OPEN)
> : -		return 0;
> : +		goto ok;
> : 
> : ...
> : 
> :  		}
> :  		return -EPERM;
> :  	}
> : +ok:
> :  	return security_syslog(type);
> :  }
> 
> 
> Means that we will now call security_syslog() for SYSLOG_ACTION_CLOSE,
> SYSLOG_ACTION_READ, SYSLOG_ACTION_READ_ALL, etc.
> 
> That's new behaviour and it may be wrong.  Why should
> check_syslog_permissions() call security_syslog() for anything other
> than SYSLOG_ACTION_OPEN?

But it isn't new behaviour.
Previously security_syslog() was called from do_syslog(), 
now it will be called from check_syslog_permissions()

from_file = true == SYSLOG_FROM_PROC is set in kmsg_open/release/read/pool()
only. These functions use do_syslog() that had called security_syslog() 
right after return from check_syslog_permissions().

sys_syslog() calls this security hook for any action and does it long time ago.

The only place where behaviour is changed, where hook was _NOT_called is
check_syslog_permissions(SYSLOG_ACTION_READ_ALL) calls from devkmsg_open()
and pstore_check_syslog_permissions().
But they does it only if dmesg_restrict is set, that looks wrong for me,
because dmesg_restict should add restrictions but do not remove existing ones.

So I do not see any new problems here.
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