dummynet: Avoid an out-of-bounds read in do_config()

do_config() processes a buffer of variable-length dummynet commands.
The loop which processes this buffer loads the fixed-length header
before checking whether there are any bytes left to read, so it performs
a 4-byte read past the end of the buffer before terminating.

Restructure the loop to avoid this.

Reported by:	Jenkins (KASAN job)
Reviewed by:	kp
Sponsored by:	The FreeBSD Foundation

(cherry picked from commit d5ea04ee7ba6c7cd8e0918a080caf5f2c8fb3955)

1 files changed, 3 insertions, 1 deletions

diff --git a/sys/netpfil/ipfw/ip_dummynet.c b/sys/netpfil/ipfw/ip_dummynet.c
index 5a88a803e88d..57dbcb3c9a35 100644
--- a/sys/netpfil/ipfw/ip_dummynet.c
+++ b/sys/netpfil/ipfw/ip_dummynet.c
@@ -2012,7 +2012,9 @@ do_config(void *p, int l)
 	}
 	arg = NULL;
 	dn = NULL;
-	for (off = 0; l >= sizeof(o); memcpy(&o, (char *)p + off, sizeof(o))) {
+	off = 0;
+	while (l >= sizeof(o)) {
+		memcpy(&o, (char *)p + off, sizeof(o));
 		if (o.len < sizeof(o) || l < o.len) {
 			D("bad len o.len %d len %d", o.len, l);
 			err = EINVAL;