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>

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 48586ac3a62e3691c4950c1c72d904e6ebf036af..10909c571494893ffba0643784a115ab04267ac1 100644 (file)
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -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;
                }