Re: [PATCH v2] kmsg: Honor dmesg_restrict sysctl on /dev/kmsg

[Josh Boyer]

On Tue, Apr 09, 2013 at 11:48:20AM -0400, Josh Boyer wrote:
> The dmesg_restrict sysctl currently covers the syslog method for access
> dmesg, however /dev/kmsg isn't covered by the same protections.  Most
> people haven't noticed because util-linux dmesg(1) defaults to using the
> syslog method for access in older versions.  With util-linux dmesg(1)
> defaults to reading directly from /dev/kmsg.
> 
> Fix this by reworking all of the access methods to use the
> check_syslog_permissions function and adding checks to devkmsg_open and
> devkmsg_read.
> 
> This fixes https://bugzilla.redhat.com/show_bug.cgi?id=903192

So this does fix that bug.  But then it introduced this one:

https://bugzilla.redhat.com/show_bug.cgi?id=952655

Basically, dmesg(1) now always falls back to using the syslog interface
instead of /dev/kmsg because nothing is granting CAP_SYSLOG for normal
users.  That was somewhat intentional based on the feedback from Kees
and Eric, but it does present a problem.

If we want to keep the existing open behavior for /dev/kmsg, and still
honor dmesg_restrict, we basically need it to fail in devkmsg_read.
With the current functions we have, that won't work so we'll either need
to hack that up or just have devkmsg_read call syslog_action_restricted
instead.

josh

> Reported-by: Christian Kujau <lists@xxxxxxxxxxxxxxx>
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Eric Paris <eparis@xxxxxxxxxx>
> Signed-off-by: Josh Boyer <jwboyer@xxxxxxxxxx>
> ---
> 
>  v2: Rework patch based on code from Eric Paris, add check in devkmsg_read as
>      suggested by Kees Cook.
> 
>  kernel/printk.c | 91 +++++++++++++++++++++++++++++----------------------------
>  1 file changed, 47 insertions(+), 44 deletions(-)
> 
> diff --git a/kernel/printk.c b/kernel/printk.c
> index abbdd9e..5541095 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -368,6 +368,46 @@ static void log_store(int facility, int level,
>  	log_next_seq++;
>  }
>  
> +#ifdef CONFIG_SECURITY_DMESG_RESTRICT
> +int dmesg_restrict = 1;
> +#else
> +int dmesg_restrict;
> +#endif
> +
> +static int syslog_action_restricted(int type)
> +{
> +	if (dmesg_restrict)
> +		return 1;
> +	/* Unless restricted, we allow "read all" and "get buffer size" for everybody */
> +	return type != SYSLOG_ACTION_READ_ALL && type != SYSLOG_ACTION_SIZE_BUFFER;
> +}
> +
> +static int check_syslog_permissions(int type, bool from_file)
> +{
> +	/*
> +	 * If this is from /proc/kmsg and we've already opened it, then we've
> +	 * already done the capabilities checks at open time.
> +	 */
> +	if (from_file && type != SYSLOG_ACTION_OPEN)
> +		goto ok;
> +
> +	if (syslog_action_restricted(type)) {
> +		if (capable(CAP_SYSLOG))
> +			goto ok;
> +		/* For historical reasons, accept CAP_SYS_ADMIN too, with a warning */
> +		if (capable(CAP_SYS_ADMIN)) {
> +			printk_once(KERN_WARNING "%s (%d): "
> +				 "Attempt to access syslog with CAP_SYS_ADMIN "
> +				 "but no CAP_SYSLOG (deprecated).\n",
> +				 current->comm, task_pid_nr(current));
> +			goto ok;
> +		}
> +		return -EPERM;
> +	}
> +ok:
> +	return security_syslog(type);
> +}
> +
>  /* /dev/kmsg - userspace message inject/listen interface */
>  struct devkmsg_user {
>  	u64 seq;
> @@ -443,10 +483,16 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  	char cont = '-';
>  	size_t len;
>  	ssize_t ret;
> +	int err;
>  
>  	if (!user)
>  		return -EBADF;
>  
> +	err = check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
> +		SYSLOG_FROM_FILE);
> +	if (err)
> +		return err;
> +
>  	ret = mutex_lock_interruptible(&user->lock);
>  	if (ret)
>  		return ret;
> @@ -624,7 +670,7 @@ static int devkmsg_open(struct inode *inode, struct file *file)
>  	if ((file->f_flags & O_ACCMODE) == O_WRONLY)
>  		return 0;
>  
> -	err = security_syslog(SYSLOG_ACTION_READ_ALL);
> +	err = check_syslog_permissions(SYSLOG_ACTION_OPEN, SYSLOG_FROM_FILE);
>  	if (err)
>  		return err;
>  
> @@ -817,45 +863,6 @@ static inline void boot_delay_msec(int level)
>  }
>  #endif
>  
> -#ifdef CONFIG_SECURITY_DMESG_RESTRICT
> -int dmesg_restrict = 1;
> -#else
> -int dmesg_restrict;
> -#endif
> -
> -static int syslog_action_restricted(int type)
> -{
> -	if (dmesg_restrict)
> -		return 1;
> -	/* Unless restricted, we allow "read all" and "get buffer size" for everybody */
> -	return type != SYSLOG_ACTION_READ_ALL && type != SYSLOG_ACTION_SIZE_BUFFER;
> -}
> -
> -static int check_syslog_permissions(int type, bool from_file)
> -{
> -	/*
> -	 * If this is from /proc/kmsg and we've already opened it, then we've
> -	 * already done the capabilities checks at open time.
> -	 */
> -	if (from_file && type != SYSLOG_ACTION_OPEN)
> -		return 0;
> -
> -	if (syslog_action_restricted(type)) {
> -		if (capable(CAP_SYSLOG))
> -			return 0;
> -		/* For historical reasons, accept CAP_SYS_ADMIN too, with a warning */
> -		if (capable(CAP_SYS_ADMIN)) {
> -			printk_once(KERN_WARNING "%s (%d): "
> -				 "Attempt to access syslog with CAP_SYS_ADMIN "
> -				 "but no CAP_SYSLOG (deprecated).\n",
> -				 current->comm, task_pid_nr(current));
> -			return 0;
> -		}
> -		return -EPERM;
> -	}
> -	return 0;
> -}
> -
>  #if defined(CONFIG_PRINTK_TIME)
>  static bool printk_time = 1;
>  #else
> @@ -1131,10 +1138,6 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
>  	if (error)
>  		goto out;
>  
> -	error = security_syslog(type);
> -	if (error)
> -		return error;
> -
>  	switch (type) {
>  	case SYSLOG_ACTION_CLOSE:	/* Close log */
>  		break;
> -- 
> 1.8.1.4
> 
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