Message-ID: <1390864354.20150.57.camel@joe-AO722>
Subject: Re: [PATCH] vsprintf: BUG on %n
From: Joe Perches <joe@perches.com>
To: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Jiri Kosina <jkosina@suse.cz>, Al Viro <viro@zeniv.linux.org.uk>,
        Olof Johansson <olof@lixom.net>,
        Stepan Moskovchenko <stepanm@codeaurora.org>,
        Daniel Borkmann <dborkman@redhat.com>, Ryan Mallon <rmallon@gmail.com>
Date: Mon, 27 Jan 2014 15:12:34 -0800
In-Reply-To: <20140127230326.GA877@www.outflux.net>
References: <20140127230326.GA877@www.outflux.net>
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Sender: linux-kernel-owner@vger.kernel.org

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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