Re: [PATCH 1/7] printk: guard the amount written per line by devkmsg_read()

[Petr Mladek]

On Thu 2015-05-14 12:06:20, Tejun Heo wrote:
> devkmsg_read() uses 8k buffer and assumes that the formatted output
> message won't overrun which seems safe given LOG_LINE_MAX, the current
> use of dict and the escaping method being used; however, we're
> planning to use devkmsg formatting wider and accounting for the buffer
> size properly isn't that complicated.
> 
> This patch defines CONSOLE_EXT_LOG_MAX as 8192 and updates
> devkmsg_read() so that it limits output accordingly.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

Reviewed-by: Petr Mladek <pmladek@xxxxxxx>

It is more secure now.

Best Regards,
Petr


> Cc: Kay Sievers <kay@xxxxxxxx>
> Cc: Petr Mladek <pmladek@xxxxxxx>
> ---
>  include/linux/printk.h |  2 ++
>  kernel/printk/printk.c | 34 ++++++++++++++++++++++------------
>  2 files changed, 24 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 9b30871..58b1fec 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -30,6 +30,8 @@ static inline const char *printk_skip_level(const char *buffer)
>  	return buffer;
>  }
>  
> +#define CONSOLE_EXT_LOG_MAX	8192
> +
>  /* printk's without a loglevel use this.. */
>  #define MESSAGE_LOGLEVEL_DEFAULT CONFIG_MESSAGE_LOGLEVEL_DEFAULT
>  
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index c099b08..a115490 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -505,6 +505,11 @@ int check_syslog_permissions(int type, bool from_file)
>  	return security_syslog(type);
>  }
>  
> +static void append_char(char **pp, char *e, char c)
> +{
> +	if (*pp < e)
> +		*(*pp)++ = c;
> +}
>  
>  /* /dev/kmsg - userspace message inject/listen interface */
>  struct devkmsg_user {
> @@ -512,7 +517,7 @@ struct devkmsg_user {
>  	u32 idx;
>  	enum log_flags prev;
>  	struct mutex lock;
> -	char buf[8192];
> +	char buf[CONSOLE_EXT_LOG_MAX];
>  };
>  
>  static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from)
> @@ -570,6 +575,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  {
>  	struct devkmsg_user *user = file->private_data;
>  	struct printk_log *msg;
> +	char *p, *e;
>  	u64 ts_usec;
>  	size_t i;
>  	char cont = '-';
> @@ -579,6 +585,9 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  	if (!user)
>  		return -EBADF;
>  
> +	p = user->buf;
> +	e = user->buf + sizeof(user->buf);
> +
>  	ret = mutex_lock_interruptible(&user->lock);
>  	if (ret)
>  		return ret;
> @@ -625,9 +634,9 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  		 ((user->prev & LOG_CONT) && !(msg->flags & LOG_PREFIX)))
>  		cont = '+';
>  
> -	len = sprintf(user->buf, "%u,%llu,%llu,%c;",
> -		      (msg->facility << 3) | msg->level,
> -		      user->seq, ts_usec, cont);
> +	p += scnprintf(p, e - p, "%u,%llu,%llu,%c;",
> +		       (msg->facility << 3) | msg->level,
> +		       user->seq, ts_usec, cont);
>  	user->prev = msg->flags;
>  
>  	/* escape non-printable characters */
> @@ -635,11 +644,11 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  		unsigned char c = log_text(msg)[i];
>  
>  		if (c < ' ' || c >= 127 || c == '\\')
> -			len += sprintf(user->buf + len, "\\x%02x", c);
> +			p += scnprintf(p, e - p, "\\x%02x", c);
>  		else
> -			user->buf[len++] = c;
> +			append_char(&p, e, c);
>  	}
> -	user->buf[len++] = '\n';
> +	append_char(&p, e, '\n');
>  
>  	if (msg->dict_len) {
>  		bool line = true;
> @@ -648,30 +657,31 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  			unsigned char c = log_dict(msg)[i];
>  
>  			if (line) {
> -				user->buf[len++] = ' ';
> +				append_char(&p, e, ' ');
>  				line = false;
>  			}
>  
>  			if (c == '\0') {
> -				user->buf[len++] = '\n';
> +				append_char(&p, e, '\n');
>  				line = true;
>  				continue;
>  			}
>  
>  			if (c < ' ' || c >= 127 || c == '\\') {
> -				len += sprintf(user->buf + len, "\\x%02x", c);
> +				p += scnprintf(p, e - p, "\\x%02x", c);
>  				continue;
>  			}
>  
> -			user->buf[len++] = c;
> +			append_char(&p, e, c);
>  		}
> -		user->buf[len++] = '\n';
> +		append_char(&p, e, '\n');
>  	}
>  
>  	user->idx = log_next(user->idx);
>  	user->seq++;
>  	raw_spin_unlock_irq(&logbuf_lock);
>  
> +	len = p - user->buf;
>  	if (len > count) {
>  		ret = -EINVAL;
>  		goto out;
> -- 
> 2.1.0
> 
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