Re: [PATCH] vsprintf: fix possible NULL pointer deref in vsnprintf()

[Kees Cook]

On Fri, Jan 06, 2023 at 12:14:57PM -0500, Steven Rostedt wrote:
> On Fri, 6 Jan 2023 16:49:46 +0100
> Petr Mladek <pmladek@xxxxxxxx> wrote:
> 
> > > Index: linux/lib/vsprintf.c
> > > ===================================================================
> > > --- linux.orig/lib/vsprintf.c
> > > +++ linux/lib/vsprintf.c
> > > @@ -2738,6 +2738,15 @@ int vsnprintf(char *buf, size_t size, co
> > >  	if (WARN_ON_ONCE(size > INT_MAX))
> > >  		return 0;
> > >  
> > > +	/*
> > > +	 * C99 allows @buf to be NULL when @size is 0. We treat such NULL as if
> > > +	 * @buf pointed to 0-sized buffer, so we can both avoid a NULL pointer
> > > +	 * dereference and still return # of characters that would be written
> > > +	 * if @buf pointed to a valid buffer...
> > > +	 */
> > > +	if (!buf)
> > > +		size = 0;  
> > 
> > It makes sense except that it would hide bugs. It should print a
> > warning, for example:
> 
> I agree. This is a bug, and should not be quietly ignored.

Yup.

> 
> > 
> > 	char *err_msg;
> > 
> > 	err_msg = check_pointer_msg(buf);
> > 	if (err_msg) {
> > 		WARN_ONCE(1, "Invalid buffer passed to vsnprintf(). Trying to continue with 0 length limit\n");
> > 		size = 0;
> > 	}
> 
> 	if (WARN_ONCE(err_msg, "Invalid buffer passed to vsnprintf(). Trying to continue with 0 length limit\n"))
> 		size = 0;

Also good. Please CC me for an Ack when this is a full patch. :)

-Kees

-- 
Kees Cook