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

[Josh Boyer]

On Wed, Apr 24, 2013 at 01:58:35PM -0400, Josh Boyer wrote:
> On Wed, Apr 24, 2013 at 07:44:33PM +0200, Kay Sievers wrote:
> > On Tue, Apr 9, 2013 at 6:33 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > > On Tue, Apr 9, 2013 at 8:48 AM, Josh Boyer <jwboyer@xxxxxxxxxx> 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
> > >>
> > >> Reported-by: Christian Kujau <lists@xxxxxxxxxxxxxxx>
> > >> CC: stable@xxxxxxxxxxxxxxx
> > >> Signed-off-by: Eric Paris <eparis@xxxxxxxxxx>
> > >> Signed-off-by: Josh Boyer <jwboyer@xxxxxxxxxx>
> > >
> > > Thanks!
> > >
> > > Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > 
> > If that's the version currently in Fedora, we just cannot do this.
> >    https://bugzilla.redhat.com/show_bug.cgi?id=952655
> > 
> > /dev/kmsg is supposed, and was added, to be a sane alternative to
> > syslog(). It is already used in dmesg(1) which is now broken with this
> > patch.
> > 
> > The access rules for /dev/kmsg should follow the access rules of
> > syslog(), and not be any stricter.
> 
> I haven't tested it yet, but I think something like this should work
> while still honoring dmesg_restrict.  I'll test it out while the rest
> of you debate things.

Yeah, that seems to work.  So, comments or Reviewed-by/Acked-by on it
would be welcome.

josh

> From: Josh Boyer <jwboyer@xxxxxxxxxx>
> Date: Tue, 9 Apr 2013 11:08:13 -0400
> Subject: [PATCH v3] kmsg: Honor dmesg_restrict sysctl on /dev/kmsg
> 
> 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
> 
> Reported-by: Christian Kujau <lists@xxxxxxxxxxxxxxx>
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Eric Paris <eparis@xxxxxxxxxx>
> Signed-off-by: Josh Boyer <jwboyer@xxxxxxxxxx>
> ---
>  v3: Allow devkmsg_open to work without CAP_SYSLOG, but still make
>      devkmsg_read honor dmesg_restrict
> 
>  kernel/printk.c | 91 +++++++++++++++++++++++++++++----------------------------
>  1 file changed, 47 insertions(+), 44 deletions(-)
> 
> diff --git a/kernel/printk.c b/kernel/printk.c
> index abbdd9e..2d7be05 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_CALL);
> +	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_READ_ALL, 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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