Subject: Re: [PATCH] SUNRPC: Prevent kernel stack corruption on long values of flush

From: "J. Bruce Fields" <[email-masked]>
Date: Wed, 18 Jul 2012 13:39:13 -0400

On Tue, Jul 17, 2012 at 12:01:26AM +0200, Sasha Levin wrote:
> The buffer size in read_flush() is too small for the longest possible values
> for it. This can lead to a kernel stack corruption:

Thanks!

> 
> diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
> index 2afd2a8..f86d95e 100644
> --- a/net/sunrpc/cache.c
> +++ b/net/sunrpc/cache.c
> @@ -1409,11 +1409,11 @@ static ssize_t read_flush(struct file *file, char __user *buf,
>  			  size_t count, loff_t *ppos,
>  			  struct cache_detail *cd)
>  {
> -	char tbuf[20];
> +	char tbuf[22];

I wonder how common this sort of calculation is in the kernel?  It might
provide some peace of mind to be able to write this something like

	char tbuf[MAXLEN_BASE10_UL + 2]  /* + 2 for final "\n\0" */

--b.

>  	unsigned long p = *ppos;
>  	size_t len;
>  
> -	sprintf(tbuf, "%lu\n", convert_to_wallclock(cd->flush_time));
> +	snprintf(tbuf, sizeof(tbuf), "%lu\n", convert_to_wallclock(cd->flush_time));
>  	len = strlen(tbuf);
>  	if (p >= len)
>  		return 0;
> -- 
> 1.7.8.6
>