net/packet: check length in getsockopt() called with PACKET_HDRLEN

In the case getsockopt() is called with PACKET_HDRLEN and optlen < 4 |val| remains uninitialized and the syscall may behave differently depending on its value, and even copy garbage to userspace on certain architectures. To fix this we now return -EINVAL if optlen is too small.

This bug has been detected with KMSAN.

---
The previous versions of this patch were called "net/packet: initialize val in packet_getsockopt()"

v3: - change patch summary, return -EINVAL for optlen < sizeof(int)
v2: - if len < sizeof(int), make it 0

---
 net/packet/af_packet.c | 2 ++
 1 file changed, 2 insertions(+)

index 8489beff5c25..ea81ccf3c7d6 100644
@@ -3836,6 +3836,8 @@ static int packet_getsockopt(struct socket *sock, int level, int optname,
 	case PACKET_HDRLEN:
 		if (len > sizeof(int))
 			len = sizeof(int);
+		if (len < sizeof(int))
+			return -EINVAL;
 		if (copy_from_user(&val, optval, len))
 			return -EFAULT;
 		switch (val) {

2.13.0.rc0.306.g87b477812d-goog

From: Alexander Potapenko <[email protected]>
Date: Tue, 25 Apr 2017 18:51:46 +0200

Applied, thanks.