Re: [PATCH] kmsg: Honor dmesg_restrict sysctl on /dev/kmsg

[Kees Cook <keescook@chromium.org>]

On Fri, Feb 22, 2013 at 01:18:57PM -0500, Josh Boyer wrote:
> Originally, the addition of dmesg_restrict covered both the syslog
> method of accessing dmesg, as well as /dev/kmsg itself.  This was done
> indirectly by security_syslog calling cap_syslog before doing any LSM
> checks.
> 
> However, commit 12b3052c3ee (capabilities/syslog: open code cap_syslog
> logic to fix build failure) moved the code around and pushed the checks
> into the caller itself.  That seems to have inadvertently dropped the
> checks for dmesg_restrict on /dev/kmsg.  Most people haven't noticed
> because util-linux dmesg(1) defaults to using the syslog method for
> access in older versions.  With util-linux 2.22 and a kernel newer than
> 3.5, dmesg(1) defaults to reading directly from /dev/kmsg.
> 
> Fix this by making an explicit check in the devkmsg_open function.
> 
> This fixes https://bugzilla.redhat.com/show_bug.cgi?id=903192
> 
> Reported-by: Christian Kujau <lists@nerdbynature.de>
> CC: stable@vger.kernel.org
> Signed-off-by: Josh Boyer <jwboyer@redhat.com>
> ---
>  kernel/printk.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/printk.c b/kernel/printk.c
> index f24633a..398ef9a 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -615,6 +615,9 @@ static int devkmsg_open(struct inode *inode, struct file *file)
>  	struct devkmsg_user *user;
>  	int err;
>  
> +	if (dmesg_restrict && !capable(CAP_SYSLOG))
> +		return -EACCES;
> +

I think this should use check_syslog_permissions() instead, as done for
/proc/kmsg and the syslog syscall.

	err = check_syslog_permissions(SYSLOG_ACTION_OPTION, SYSLOG_FROM_FILE);
	if (err)
		return err;

And going forward we should probably think about dropping the CAP_SYS_ADMIN
backward-compat code in check_syslog_permissions.

>  	/* write-only does not need any file context */
>  	if ((file->f_flags & O_ACCMODE) == O_WRONLY)
>  		return 0;

-Kees

-- 
Kees Cook
Chrome OS Security