linux-kernel - [PATCH] vsprintf: ignore arguments to %n

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Date:	Mon, 27 Jan 2014 16:39:27 -0800
From:	Kees Cook <keescook@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ryan Mallon <rmallon@...il.com>, Jiri Kosina <jkosina@...e.cz>,
	Joe Perches <joe@...ches.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Olof Johansson <olof@...om.net>,
	Stepan Moskovchenko <stepanm@...eaurora.org>,
	Daniel Borkmann <dborkman@...hat.com>
Subject: [PATCH] vsprintf: ignore arguments to %n

If arguments are consumed without output when encountering %n, it
could be used to benefit or improve information leak attacks that were
exposed via a limited size buffer. Since %n is not used by the kernel,
there is no reason to make an info leak attack any easier.

Signed-off-by: Kees Cook <keescook@...omium.org>
Cc: stable@...r.kernel.org # 3.13+
---
 lib/vsprintf.c |   10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 185b6d300ebc..9d5c48b705f9 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1735,14 +1735,10 @@ 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,
+			 * when encountering %n, ignore the arguments.
 			 */
-			void *skip_arg;
-
-			WARN_ONCE(1, "Please remove ignored %%n in '%s'\n",
-					old_fmt);
-
-			skip_arg = va_arg(args, void *);
+			WARN_ONCE(1, "Ignored %%n in '%s'\n", old_fmt);
 			break;
 		}
 
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security
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