linux-kernel - Re: [PATCH] syslog: check cap_syslog when dmesg

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Message-ID: <1291822780.3072.40.camel@localhost.localdomain>
Date:	Wed, 08 Dec 2010 10:39:40 -0500
From:	Eric Paris <eparis@...hat.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	James Morris <jmorris@...ei.org>,
	Stephen Smalley <sds@...ho.nsa.gov>, dwalsh@...hat.com,
	Kees Cook <kees.cook@...onical.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] syslog: check cap_syslog when dmesg_restrict

On Wed, 2010-12-08 at 15:19 +0000, Serge E. Hallyn wrote:
> Eric Paris pointed out that it doesn't make sense to require
> both CAP_SYS_ADMIN and CAP_SYSLOG for certain syslog actions.
> So require CAP_SYSLOG, not CAP_SYS_ADMIN, when dmesg_restrict
> is set.
> 
> (I'm also consolidating the now common error path)
> 
> Signed-off-by: Serge E. Hallyn <serge.hallyn@...onical.com>
> ---
>  Documentation/sysctl/kernel.txt |    2 +-
>  kernel/printk.c                 |   20 ++++++++++----------
>  2 files changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index 209e158..5740671 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -219,7 +219,7 @@ dmesg_restrict:
>  This toggle indicates whether unprivileged users are prevented from using
>  dmesg(8) to view messages from the kernel's log buffer.  When
>  dmesg_restrict is set to (0) there are no restrictions.  When
> -dmesg_restrict is set set to (1), users must have CAP_SYS_ADMIN to use
> +dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
>  dmesg(8).
>  
>  The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the default
> diff --git a/kernel/printk.c b/kernel/printk.c
> index 0712380..0cecba0 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -279,18 +279,12 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
>  	 * at open time.
>  	 */
>  	if (type == SYSLOG_ACTION_OPEN || !from_file) {
> -		if (dmesg_restrict && !capable(CAP_SYS_ADMIN))
> -			return -EPERM;
> +		if (dmesg_restrict && !capable(CAP_SYSLOG))
> +			goto warn; /* switch to return -EPERM after 2.6.39 */
>  		if ((type != SYSLOG_ACTION_READ_ALL &&
>  		     type != SYSLOG_ACTION_SIZE_BUFFER) &&
> -		    !capable(CAP_SYSLOG)) {
> -			/* remove after 2.6.38 */
> -			if (capable(CAP_SYS_ADMIN))
> -				WARN_ONCE(1, "Attempt to access syslog with "
> -				  "CAP_SYS_ADMIN but no CAP_SYSLOG "
> -				  "(deprecated and denied).\n");
> -			return -EPERM;
> -		}
> +		    !capable(CAP_SYSLOG))
> +			goto warn; /* switch to return -EPERM after 2.6.39 */

Doesn't this return -EPERM right now?  I think the code might be
incorrect today as well......

I thought the flow was supposed to be

if (capable(CAP_SYSLOG))
	all good
else if (capable(CAP_SYS_ADMIN))
	WARN, but still good for now
else
	EPERM

But it looks to me like the flow is

if (capable(CAP_SYSLOG))
	all good
else if (capable(CAP_SYS_ADMIN))
	WARN, EPERM
else
	EPERM

>  	}
>  
>  	error = security_syslog(type);
> @@ -434,6 +428,12 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
>  	}
>  out:
>  	return error;
> +warn:
> +	/* remove after 2.6.39 */
> +	if (capable(CAP_SYS_ADMIN))
> +		WARN_ONCE(1, "Attempt to access syslog with CAP_SYS_ADMIN "
> +		  "but no CAP_SYSLOG (deprecated and denied).\n");
> +	return -EPERM;
>  }
>  
>  SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)


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