[PATCH 3/3] vsprintf: ignore %n again

[Kees Cook <keescook@chromium.org>]

This ignores %n in printf again, as was originally documented. Implementing
%n poses a greater security risk than utility, so it should stay ignored.
To help anyone attempting to use %n, a warning will be emitted if it is
encountered.

Based on earlier patch by Joe Perches.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Joe Perches <joe@perches.com>
---
 lib/vsprintf.c |   20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 26559bd..478c30e 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1683,18 +1683,16 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
 			break;
 
 		case FORMAT_TYPE_NRCHARS: {
-			u8 qualifier = spec.qualifier;
+			/*
+			 * Since %n poses a greater security risk than
+			 * utility, ignore %n and skip its argument.
+			 */
+			void *skip_arg;
 
-			if (qualifier == 'l') {
-				long *ip = va_arg(args, long *);
-				*ip = (str - buf);
-			} else if (_tolower(qualifier) == 'z') {
-				size_t *ip = va_arg(args, size_t *);
-				*ip = (str - buf);
-			} else {
-				int *ip = va_arg(args, int *);
-				*ip = (str - buf);
-			}
+			WARN_ONCE(1, "Please remove ignored %%n in '%s'\n",
+					old_fmt);
+
+			skip_arg = va_arg(args, void *);
 			break;
 		}
 
-- 
1.7.9.5