Subject: Re: [patch v3] fix stack overflow in pktgen_if_write()

From: Nelson Elhage <[email-masked]>
Date: Thu, 28 Oct 2010 11:22:22 -0400

You've got a leak if copy_user fails.

While testing this, I realized that printk() won't print more than 1k in a
single call, anyways, so I've sent along a patch that just copies up to 1k onto
the stack, which should prevent the overflow without changing behavior or
needing a heap allocation.

- Nelson

On Thu, Oct 28, 2010 at 08:05:29AM +0200, Dan Carpenter wrote:
> Nelson Elhage says he was able to oops both amd64 and i386 test 
> machines with 8k writes to the pktgen file.  Let's just allocate the
> buffer on the heap instead of on the stack.
> 
> This can only be triggered by root so there are no security issues here.
> 
> Reported-by: Nelson Elhage <[email-masked]>
> Signed-off-by: Dan Carpenter <[email-masked]>
> ---
> v3:  just use kmalloc()
> 
> diff --git a/net/core/pktgen.c b/net/core/pktgen.c
> index 2c0df0f..c8d3620 100644
> --- a/net/core/pktgen.c
> +++ b/net/core/pktgen.c
> @@ -887,12 +887,17 @@ static ssize_t pktgen_if_write(struct file *file,
>  	i += len;
>  
>  	if (debug) {
> -		char tb[count + 1];
> +		char *tb;
> +
> +		tb = kmalloc(count + 1, GFP_KERNEL);
> +		if (!tb)
> +			return -ENOMEM;
>  		if (copy_from_user(tb, user_buffer, count))
>  			return -EFAULT;
>  		tb[count] = 0;
>  		printk(KERN_DEBUG "pktgen: %s,%lu  buffer -:%s:-\n", name,
>  		       (unsigned long)count, tb);
> +		kfree(tb);
>  	}
>  
>  	if (!strcmp(name, "min_pkt_size")) {