[Patch net] ip_gre: set dev->hard_header_len properly

[Cong Wang]

GRE tunnel has its own header_ops, ipgre_header_ops, and sets it
conditionally. When it is set, it assumes the outer IP header is
already created before ipgre_xmit().

This is not true when we send packets through a raw packet socket,
where L2 headers are supposed to be constructed by user. Packet
socket calls dev_validate_header() to validate the header. But
GRE tunnel does not set dev->hard_header_len, so that check can
be simply bypassed, therefore uninit memory could be passed down
to ipgre_xmit().

Fix this by setting dev->hard_header_len whenever sets header_ops,
as dev->hard_header_len is supposed to be the length of the header
created by dev->header_ops->create() anyway.

Reported-and-tested-by: syzbot+4a2c52677a8a1aa28...@syzkaller.appspotmail.com
Cc: William Tu <u9012...@gmail.com>
Cc: Willem de Bruijn <willemdebruijn.ker...@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangc...@gmail.com>
---
 net/ipv4/ip_gre.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 4e31f23e4117..43b62095559e 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -987,10 +987,12 @@ static int ipgre_tunnel_init(struct net_device *dev)
                                return -EINVAL;
                        dev->flags = IFF_BROADCAST;
                        dev->header_ops = &ipgre_header_ops;
+                       dev->hard_header_len = tunnel->hlen + sizeof(*iph);
                }
 #endif
        } else if (!tunnel->collect_md) {
                dev->header_ops = &ipgre_header_ops;
+               dev->hard_header_len = tunnel->hlen + sizeof(*iph);
        }
 
        return ip_tunnel_init(dev);
-- 
2.28.0