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

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Date:   Fri, 6 Jan 2023 12:14:57 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Shtylyov <s.shtylyov@....ru>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vsprintf: fix possible NULL pointer deref in
 vsnprintf()

On Fri, 6 Jan 2023 16:49:46 +0100
Petr Mladek <pmladek@...e.com> 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.

> 
> 	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;

  ;-)

-- Steve

> 
> check_pointer_msg() allows to catch even more buggy pointers. WARN()
> helps to locate the caller. WARN_ONCE() variant is used to prevent
> a potential infinite loop.
> 
> > +
> >  	str = buf;
> >  	end = buf + size;
> >