Commit 9196436a authored Nov 14, 2013 by Kees Cook Committed by Linus Torvalds Nov 15, 2013

vsprintf: ignore %n again



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 an earlier patch by Joe Perches.

Because %n was designed to write to pointers on the stack, it has been
frequently used as an attack vector when bugs are found that leak
user-controlled strings into functions that ultimately process format
strings.  While this class of bug can still be turned into an
information leak, removing %n eliminates the common method of elevating
such a bug into an arbitrary kernel memory writing primitive,
significantly reducing the danger of this class of bug.

For seq_file users that need to know the length of a written string for
padding, please see seq_setwidth() and seq_pad() instead.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Joe Perches <joe@perches.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 652586df

lib/vsprintf.c

+9 −11

Original line number	Diff line number	Diff line
		@@ -1712,18 +1712,16 @@ 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;
		}