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

From: Jim Rees <[email-masked]>
Date: Wed, 18 Jul 2012 16:00:49 -0400

J. Bruce Fields wrote:

  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" */

You could use something like:

    char tbuf[sizeof (unsigned long) * 24 / 10 + 1 + 2]; /* + 2 for final "\n\0" */

since there are roughly 10 bits for every 3 decimal digits.

But I'm obviously confused, because I don't understand why tbuf needs to be
any more than 10 + 2.