[PATCH] vsprintf: BUG on %n

[Kees Cook <keescook@chromium.org>]

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@chromium.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