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

[Steven Rostedt]

On Wed, 6 Jul 2016 15:29:24 +0200
Borislav Petkov <b...@alien8.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