bpf: fix null pointer deref in bpf_prog_test_run_xdp

syzkaller was able to generate the following XDP program ...

(18) r0 = 0x0
(61) r5 = *(u32 *)(r1 +12)
(04) (u32) r0 += (u32) 0
(95) exit

... and trigger a NULL pointer dereference in ___bpf_prog_run()
via bpf_prog_test_run_xdp() where this was attempted to run.

Reason is that recent xdp_rxq_info addition to XDP programs
updated all drivers, but not bpf_prog_test_run_xdp(), where
xdp_buff is set up. Thus when context rewriter does the deref
on the netdev it's NULL at runtime. Fix it by using xdp_rxq
from loopback dev. __netif_get_rx_queue() helper can also be
reused in various other locations later on.

Fixes: 02dd3291b2f0 ("bpf: finally expose xdp_rxq_info to XDP bpf-programs")
Reported-by: [e-mail]
Signed-off-by: Daniel Borkmann <[e-mail]>
Cc: Jesper Dangaard Brouer <[e-mail]>
Acked-by: Jesper Dangaard Brouer <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>

include/linux/netdevice.h

···
 }
 #endif
 
+static inline struct netdev_rx_queue *
+__netif_get_rx_queue(struct net_device *dev, unsigned int rxq)
+{
+	return dev->_rx + rxq;
+}
+
 #ifdef CONFIG_SYSFS
 static inline unsigned int get_netdev_rx_queue_index(
 		struct netdev_rx_queue *queue)

net/bpf/test_run.c

···
 {
 	u32 size = kattr->test.data_size_in;
 	u32 repeat = kattr->test.repeat;
+	struct netdev_rx_queue *rxqueue;
 	struct xdp_buff xdp = {};
 	u32 retval, duration;
 	void *data;
···
 	xdp.data = data + XDP_PACKET_HEADROOM + NET_IP_ALIGN;
 	xdp.data_meta = xdp.data;
 	xdp.data_end = xdp.data + size;
+
+	rxqueue = __netif_get_rx_queue(current->nsproxy->net_ns->loopback_dev, 0);
+	xdp.rxq = &rxqueue->xdp_rxq;
 
 	retval = bpf_test_run(prog, &xdp, repeat, &duration);
 	if (xdp.data != data + XDP_PACKET_HEADROOM + NET_IP_ALIGN)

tools/testing/selftests/bpf/test_verifier.c

···
 		.result = REJECT,
 	},
 	{
+		"XDP, using ifindex from netdev",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
+				    offsetof(struct xdp_md, ingress_ifindex)),
+			BPF_JMP_IMM(BPF_JLT, BPF_REG_2, 1, 1),
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+		.prog_type = BPF_PROG_TYPE_XDP,
+		.retval = 1,
+	},
+	{
 		"meta access, test1",
 		.insns = {
 			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,