linux-kernel - Re: [PATCH] vsprintf: BUG on %n

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Message-ID: <1390864354.20150.57.camel@joe-AO722>
Date:	Mon, 27 Jan 2014 15:12:34 -0800
From:	Joe Perches <joe@...ches.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Al Viro <viro@...iv.linux.org.uk>,
	Olof Johansson <olof@...om.net>,
	Stepan Moskovchenko <stepanm@...eaurora.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Ryan Mallon <rmallon@...il.com>
Subject: Re: [PATCH] vsprintf: BUG on %n

On Mon, 2014-01-27 at 15:03 -0800, Kees Cook wrote:
> Now that there has been a full release of the kernel, and all users
> of %n have been dropped, switch to %n use triggering a BUG. Ignoring
> arguments could be used to assist in information leaks if an arbitrary
> format string was under the control of an attacker.
[]
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
[]
> @@ -1735,15 +1735,12 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
>  		case FORMAT_TYPE_NRCHARS: {
>  			/*
>  			 * Since %n poses a greater security risk than
> -			 * utility, ignore %n and skip its argument.
> +			 * utility, it should not be implemented. Instead,
> +			 * BUG when encountering %n, since there are no
> +			 * legitimate users and skipping arguments could
> +			 * assist information leak attacks.
>  			 */
> -			void *skip_arg;
> -
> -			WARN_ONCE(1, "Please remove ignored %%n in '%s'\n",
> -					old_fmt);
> -
> -			skip_arg = va_arg(args, void *);
> -			break;
> +			BUG();

BUGs should be avoided where possible.

I think using BUG here isn't necessary or good.


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