path: root/lib/vsprintf.c
diff options
authorKees Cook <keescook@chromium.org>2013-11-14 14:31:58 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2013-11-15 09:32:20 +0900
commit9196436ab2f713b823a2ba2024cb69f40b2f54a5 (patch)
treed10e292c5e307d616edfa13345a6735ed3cc93c9 /lib/vsprintf.c
parent652586df95e5d76b37d07a11839126dcfede1621 (diff)
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>
Diffstat (limited to 'lib/vsprintf.c')
1 files changed, 9 insertions, 11 deletions
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 48586ac3a62e..10909c571494 100644
--- 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)
- 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 *);