[PATCH v1 net 1/5] sit: Pull header after checking skb->protocol in sit_tunnel_xmit().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

syzkaller crafted a GSO packet of ETH_P_8021AD + ETH_P_NSH and sent it
over sit0.

After nsh_gso_segment(), skb->data - skb->head was 138, on the other
hand, skb->network_header was 128.

Later, sit_tunnel_xmit() called pskb_inet_may_pull() for the skb, and
it caused underflow and triggered the warning in pskb_may_pull_reason().

  pskb_network_may_pull(skb, nhlen)
  `- pskb_may_pull(skb, skb_network_offset(skb) + nhlen)

  -> skb_network_offset(skb) + nhlen
   = skb_network_offset(skb) + 0 (because skb->protocol is not IPv4/IPv6)
   = skb_network_header(skb) - skb->data
   = skb->head + skb->network_header - skb->data
   = 128 - 138
   = -10

Let's check skb->protocol before pulling the next header in
sit_tunnel_xmit().

WARNING: CPU: 1 PID: 5300 at include/linux/skbuff.h:2740 pskb_may_pull_reason include/linux/skbuff.h:2740 [inline]
WARNING: CPU: 1 PID: 5300 at include/linux/skbuff.h:2740 pskb_may_pull include/linux/skbuff.h:2756 [inline]
WARNING: CPU: 1 PID: 5300 at include/linux/skbuff.h:2740 pskb_network_may_pull include/linux/skbuff.h:3077 [inline]
WARNING: CPU: 1 PID: 5300 at include/linux/skbuff.h:2740 pskb_inet_may_pull include/net/ip_tunnels.h:361 [inline]
WARNING: CPU: 1 PID: 5300 at include/linux/skbuff.h:2740 sit_tunnel_xmit+0x1080/0x1fd8 net/ipv6/sit.c:1068
Modules linked in:
CPU: 1 PID: 5300 Comm: syz-executor.5 Not tainted 6.8.0-12821-g537c2e91d354-dirty #11
Hardware name: linux,dummy-virt (DT)
pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : pskb_may_pull_reason include/linux/skbuff.h:2740 [inline]
pc : pskb_may_pull include/linux/skbuff.h:2756 [inline]
pc : pskb_network_may_pull include/linux/skbuff.h:3077 [inline]
pc : pskb_inet_may_pull include/net/ip_tunnels.h:361 [inline]
pc : sit_tunnel_xmit+0x1080/0x1fd8 net/ipv6/sit.c:1068
lr : pskb_may_pull_reason include/linux/skbuff.h:2740 [inline]
lr : pskb_may_pull include/linux/skbuff.h:2756 [inline]
lr : pskb_network_may_pull include/linux/skbuff.h:3077 [inline]
lr : pskb_inet_may_pull include/net/ip_tunnels.h:361 [inline]
lr : sit_tunnel_xmit+0x1080/0x1fd8 net/ipv6/sit.c:1068
sp : ffff8000aa017260
x29: ffff8000aa0173e0 x28: ffff00000cd0c000 x27: ffff000046200080
x26: ffff700015402e64 x25: 00000000fffffff6 x24: 1fffe00009835b7f
x23: dfff800000000000 x22: ffff00004c1adbf4 x21: ffff00004c1adbf8
x20: ffff00004c1adb40 x19: 1fffe00009835b7e x18: ffff800086d72d10
x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000001
x14: 1fffe0000a15c217 x13: 0000000000000000 x12: ffff8000aa017320
x11: 0000000000040000 x10: 0000000000000b86 x9 : ffff8000a2639000
x8 : 0000000000000b87 x7 : 0000000000000001 x6 : 0000000000000001
x5 : ffff8000aa016c98 x4 : ffff800086d8b120 x3 : ffff800083f7f7d4
x2 : 0000000000000001 x1 : 00000000fffffff6 x0 : 0000000000000000
Call trace:
 pskb_may_pull_reason include/linux/skbuff.h:2740 [inline]
 pskb_may_pull include/linux/skbuff.h:2756 [inline]
 pskb_network_may_pull include/linux/skbuff.h:3077 [inline]
 pskb_inet_may_pull include/net/ip_tunnels.h:361 [inline]
 sit_tunnel_xmit+0x1080/0x1fd8 net/ipv6/sit.c:1068
 __netdev_start_xmit include/linux/netdevice.h:4903 [inline]
 netdev_start_xmit include/linux/netdevice.h:4917 [inline]
 xmit_one net/core/dev.c:3531 [inline]
 dev_hard_start_xmit+0x1a0/0x370 net/core/dev.c:3547
 __dev_queue_xmit+0x12e8/0x2b04 net/core/dev.c:4335
 dev_queue_xmit include/linux/netdevice.h:3091 [inline]
 packet_xmit+0x6c/0x31c net/packet/af_packet.c:276
 packet_snd net/packet/af_packet.c:3081 [inline]
 packet_sendmsg+0x3818/0x4e50 net/packet/af_packet.c:3113
 sock_sendmsg_nosec net/socket.c:730 [inline]
 __sock_sendmsg net/socket.c:745 [inline]
 __sys_sendto+0x324/0x470 net/socket.c:2191
 __do_sys_sendto net/socket.c:2203 [inline]
 __se_sys_sendto net/socket.c:2199 [inline]
 __arm64_sys_sendto+0xd8/0xf8 net/socket.c:2199
 __invoke_syscall arch/arm64/kernel/syscall.c:34 [inline]
 invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:48
 el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:133
 do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:152
 el0_svc+0x30/0x78 arch/arm64/kernel/entry-common.c:712
 el0t_64_sync_handler+0x84/0xfc arch/arm64/kernel/entry-common.c:730
 el0t_64_sync+0x190/0x194 arch/arm64/kernel/entry.S:598

Reported-by: syzkaller <syzkaller@googlegroups.com>
Fixes: cb9f1b783850 ("ip: validate header length on virtual device xmit")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
 net/ipv6/sit.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
index 655c9b1a19b8..b83c28369d46 100644
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -1065,14 +1065,17 @@ static netdev_tx_t sit_tunnel_xmit__(struct sk_buff *skb,
 static netdev_tx_t sit_tunnel_xmit(struct sk_buff *skb,
 				   struct net_device *dev)
 {
-	if (!pskb_inet_may_pull(skb))
-		goto tx_err;
-
 	switch (skb->protocol) {
 	case htons(ETH_P_IP):
+		if (!pskb_network_may_pull(skb, sizeof(struct iphdr)))
+			goto tx_err;
+
 		sit_tunnel_xmit__(skb, dev, IPPROTO_IPIP);
 		break;
 	case htons(ETH_P_IPV6):
+		if (!pskb_network_may_pull(skb, sizeof(struct ipv6hdr)))
+			goto tx_err;
+
 		ipip6_tunnel_xmit(skb, dev);
 		break;
 #if IS_ENABLED(CONFIG_MPLS)
-- 
2.30.2