net: bridge: xmit: make sure we have at least eth header len bytes

syzbot triggered an uninit value[1] error in bridge device's xmit path
by sending a short (less than ETH_HLEN bytes) skb. To fix it check if
we can actually pull that amount instead of assuming.

Tested with dropwatch:
 drop at: br_dev_xmit+0xb93/0x12d0 [bridge] (0xffffffffc06739b3)
 origin: software
 timestamp: Mon May 13 11:31:53 2024 778214037 nsec
 protocol: 0x88a8
 length: 2
 original length: 2
 drop reason: PKT_TOO_SMALL

[1]
BUG: KMSAN: uninit-value in br_dev_xmit+0x61d/0x1cb0 net/bridge/br_device.c:65
 br_dev_xmit+0x61d/0x1cb0 net/bridge/br_device.c:65
 __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+0x247/0xa20 net/core/dev.c:3547
 __dev_queue_xmit+0x34db/0x5350 net/core/dev.c:4341
 dev_queue_xmit include/linux/netdevice.h:3091 [inline]
 __bpf_tx_skb net/core/filter.c:2136 [inline]
 __bpf_redirect_common net/core/filter.c:2180 [inline]
 __bpf_redirect+0x14a6/0x1620 net/core/filter.c:2187
 ____bpf_clone_redirect net/core/filter.c:2460 [inline]
 bpf_clone_redirect+0x328/0x470 net/core/filter.c:2432
 ___bpf_prog_run+0x13fe/0xe0f0 kernel/bpf/core.c:1997
 __bpf_prog_run512+0xb5/0xe0 kernel/bpf/core.c:2238
 bpf_dispatcher_nop_func include/linux/bpf.h:1234 [inline]
 __bpf_prog_run include/linux/filter.h:657 [inline]
 bpf_prog_run include/linux/filter.h:664 [inline]
 bpf_test_run+0x499/0xc30 net/bpf/test_run.c:425
 bpf_prog_test_run_skb+0x14ea/0x1f20 net/bpf/test_run.c:1058
 bpf_prog_test_run+0x6b7/0xad0 kernel/bpf/syscall.c:4269
 __sys_bpf+0x6aa/0xd90 kernel/bpf/syscall.c:5678
 __do_sys_bpf kernel/bpf/syscall.c:5767 [inline]
 __se_sys_bpf kernel/bpf/syscall.c:5765 [inline]
 __x64_sys_bpf+0xa0/0xe0 kernel/bpf/syscall.c:5765
 x64_sys_call+0x96b/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:322
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+a63a1f6a062033cf0f40@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a63a1f6a062033cf0f40Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_device.c

@@ -27,6 +27,7 @@ EXPORT_SYMBOL_GPL(nf_br_ops);
 /* net device transmit always called with BH disabled */
 netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 {
+	enum skb_drop_reason reason = pskb_may_pull_reason(skb, ETH_HLEN);
 	struct net_bridge_mcast_port *pmctx_null = NULL;
 	struct net_bridge *br = netdev_priv(dev);
 	struct net_bridge_mcast *brmctx = &br->multicast_ctx;
@@ -38,6 +39,11 @@ netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 	const unsigned char *dest;
 	u16 vid = 0;
 
+	if (unlikely(reason != SKB_NOT_DROPPED_YET)) {
+		kfree_skb_reason(skb, reason);
+		return NETDEV_TX_OK;
+	}
+
 	memset(skb->cb, 0, sizeof(struct br_input_skb_cb));
 	br_tc_skb_miss_set(skb, false);