linux-kernel - Re: [PATCH -v3.1 2/2] printk: Add kernel parameter to control writes to /dev/kmsg

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Date:	Wed, 6 Jul 2016 13:52:11 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Franck Bui <fbui@...e.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH -v3.1 2/2] printk: Add kernel parameter to control
 writes to /dev/kmsg

On Wed, 6 Jul 2016 15:29:24 +0200
Borislav Petkov <bp@...en8.de> wrote:


> @@ -800,19 +863,24 @@ static int devkmsg_open(struct inode *inode, struct file *file)
>  	struct devkmsg_user *user;
>  	int err;
>  
> -	/* write-only does not need any file context */
> -	if ((file->f_flags & O_ACCMODE) == O_WRONLY)
> -		return 0;
> +	if (devkmsg_log & DEVKMSG_LOG_MASK_OFF)
> +		return -EPERM;
>  
> -	err = check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
> -				       SYSLOG_FROM_READER);
> -	if (err)
> -		return err;
> +	/* write-only does not need any file context */
> +	if ((file->f_flags & O_ACCMODE) != O_WRONLY) {
> +		err = check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
> +					       SYSLOG_FROM_READER);
> +		if (err)
> +			return err;
> +	}
>  
>  	user = kmalloc(sizeof(struct devkmsg_user), GFP_KERNEL);
>  	if (!user)
>  		return -ENOMEM;
>  
> +	ratelimit_default_init(&user->rs);
> +	ratelimit_set_flags(&user->rs, RATELIMIT_MSG_ON_RELEASE);
> +
>  	mutex_init(&user->lock);
>  
>  	raw_spin_lock_irq(&logbuf_lock);
> @@ -831,6 +899,8 @@ static int devkmsg_release(struct inode *inode, struct file *file)
>  	if (!user)
>  		return 0;
>  
> +	ratelimit_state_exit(&user->rs);
> +
>  	mutex_destroy(&user->lock);
>  	kfree(user);
>  	return 0;
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c

Hmm, this does nothing to stop user space from doing the following:

while :; do echo '5,0,0,-;hello' > /dev/kmsg; done

But at least it's a start.

-- Steve