linux-kernel - Re: [PATCH 1/1] lib/vsprintf: Implement ssprintf() to catch truncated strings

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Message-ID: <20240129095237.GC1708181@google.com>
Date: Mon, 29 Jan 2024 09:52:37 +0000
From: Lee Jones <lee@...nel.org>
To: David Laight <David.Laight@...lab.com>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Crutcher Dunnavant <crutcher+kernel@...astacks.com>,
	Juergen Quade <quade@...r.de>
Subject: Re: [PATCH 1/1] lib/vsprintf: Implement ssprintf() to catch
 truncated strings

On Mon, 29 Jan 2024, David Laight wrote:

> ...
> > > I'm sure that the safest return for 'truncated' is the buffer length.
> > > The a series of statements like:
> > > 	buf += xxx(buf, buf_end - buf, .....);
> > > can all be called with a single overflow check at the end.
> > >
> > > Forget the check, and the length just contains a trailing '\0'
> > > which might cause confusion but isn't going to immediately
> > > break the world.
> > 
> > snprintf() does this and has been proven to cause buffer-overflows.
> > There have been multiple articles authored describing why using
> > snprintf() is not generally a good idea for the masses including the 2
> > linked in the commit message:
> 
> snprintf() returns the number of bytes that would have been output [1].
> I'm not suggesting that, or not terminating the buffer.
> Just returning the length including the '\0' (unless length was zero).
> This still lets the code check for overflow but isn't going to
> generate a pointer outside the buffer if used to update a pointer.

I see.  Well I'm not married to my solution.  However, I am convinced
that the 2 solutions currently offered can be improved upon.  If you or
anyone else has a better solution, I'd be more than happy to implement
and switch to it.

Let me have a think about the solution you suggest and get back to you.

> [1] I'm pretty certain this is because the original libc version
> of sprintf() allocated a FILE structure on stack (fully buffered)
> and called fprintf().
> snprintf() would have been done the same way but with something
> to stop the buffer being flushed.

Interesting.  Thanks for the background.

-- 
Lee Jones [李琼斯]