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Subject[PATCH] vsprintf: BUG on %n
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


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