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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140127230326.GA877@www.outflux.net>
Date:	Mon, 27 Jan 2014 15:03:26 -0800
From:	Kees Cook <keescook@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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>,
	Ryan Mallon <rmallon@...il.com>
Subject: [PATCH] vsprintf: BUG on %n

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.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 lib/vsprintf.c |   13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 185b6d300ebc..a27fd7f61325 100644
--- 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();
 		}
 
 		default:
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/