Re: [PATCH] tty: prevent DOS in the flush_to_ldisc

[Jiri Olsa]

hi, any feedback?

thanks,
jirka

On Wed, Oct 27, 2010 at 09:10:51AM +0200, Jiri Olsa wrote:
> hi,
> 
> there's a small window inside the flush_to_ldisc function,
> where the tty is unlocked and calling ldisc's receive_buf
> function. If in this window new buffer is added to the tty,
> the processing might never leave the flush_to_ldisc function.
> 
> This scenario will hog the cpu, causing other tty processing
> starving, and making it impossible to interface the computer
> via tty.
> 
> I was able to exploit this via pty interface by sending only
> control characters to the master input, causing the flush_to_ldisc
> to be scheduled, but never actually generate any output.
> 
> To reproduce, please run multiple instances of following code.
> 
> ---
> #define _XOPEN_SOURCE
> #include <stdlib.h>
> #include <stdio.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> 
> int main(int argc, char **argv)
> {
> 	int i, slave, master = getpt();
> 	char buf[8192];
> 
> 	sprintf(buf, "%s", ptsname(master));
> 	grantpt(master);
> 	unlockpt(master);
> 
> 	slave = open(buf, O_RDWR);
> 	if (slave < 0) {
> 		perror("open slave failed");
> 		return 1;
> 	}
> 
> 	for(i = 0; i < sizeof(buf); i++)
> 		buf[i] = rand() % 32;
> 
> 	while(1) {
> 		write(master, buf, sizeof(buf));
> 	}
> 
> 	return 0;
> }
> ---
> 
> The attached patch (based on -next tree) fixes this by adding threshold
> for processed data. When the threshold is reached, the current work is
> rescheduled, so another could run.
> 
> The threshold is set to the tty buffer maximum size.
> 
> wbr,
> jirka
> 
> 
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  drivers/char/tty_buffer.c |   15 ++++++++++++++-
>  include/linux/tty.h       |    1 +
>  2 files changed, 15 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/char/tty_buffer.c b/drivers/char/tty_buffer.c
> index cc1e985..7703114 100644
> --- a/drivers/char/tty_buffer.c
> +++ b/drivers/char/tty_buffer.c
> @@ -58,7 +58,7 @@ static struct tty_buffer *tty_buffer_alloc(struct tty_struct *tty, size_t size)
>  {
>  	struct tty_buffer *p;
>  
> -	if (tty->buf.memory_used + size > 65536)
> +	if (tty->buf.memory_used + size > TTY_BUFFER_MAXSIZE)
>  		return NULL;
>  	p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
>  	if (p == NULL)
> @@ -414,6 +414,7 @@ static void flush_to_ldisc(struct work_struct *work)
>  
>  	if (!test_and_set_bit(TTY_FLUSHING, &tty->flags)) {
>  		struct tty_buffer *head;
> +		int count_acc = 0;
>  		while ((head = tty->buf.head) != NULL) {
>  			int count;
>  			char *char_buf;
> @@ -436,11 +437,23 @@ static void flush_to_ldisc(struct work_struct *work)
>  				schedule_delayed_work(&tty->buf.work, 1);
>  				break;
>  			}
> +			/*
> +			 * There's a possibility tty might get new buffer
> +			 * added during the unlock window below. We could
> +			 * end up spinning in here forever hogging the CPU
> +			 * completely. To avoid this let's have a rest each
> +			 * time we process the maximum one tty can hold.
> +			 */
> +			if (count_acc > TTY_BUFFER_MAXSIZE) {
> +				schedule_delayed_work(&tty->buf.work, 1);
> +				break;
> +			}
>  			if (count > tty->receive_room)
>  				count = tty->receive_room;
>  			char_buf = head->char_buf_ptr + head->read;
>  			flag_buf = head->flag_buf_ptr + head->read;
>  			head->read += count;
> +			count_acc += count;
>  			spin_unlock_irqrestore(&tty->buf.lock, flags);
>  			disc->ops->receive_buf(tty, char_buf,
>  							flag_buf, count);
> diff --git a/include/linux/tty.h b/include/linux/tty.h
> index e500171..708e299 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -80,6 +80,7 @@ struct tty_buffer {
>   */
>  
>  #define TTY_BUFFER_PAGE	(((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~0xFF)
> +#define TTY_BUFFER_MAXSIZE	(65536)
>  
>  
>  struct tty_bufhead {
> -- 
> 1.7.1
> 
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