bpf: veth driver panics when xdp prog attached before veth_open

The following panic is observed when bringing up (veth_open) a veth device
that has an XDP program attached.

[   61.519185] kernel BUG at net/core/dev.c:6442!
[   61.519456] invalid opcode: 0000 [#1] PREEMPT SMP PTI
[   61.519752] CPU: 0 PID: 408 Comm: ip Tainted: G        W          6.1.0-rc2-185930-gd9095f92-dirty #26
[   61.520288] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
[   61.520806] RIP: 0010:napi_enable+0x3d/0x40
[   61.521077] Code: f6 f6 80 61 08 00 00 02 74 0d 48 83 bf 88 01 00 00 00 74 03 80 cd 01 48 89 d0 f0 48 0f b1 4f 10 48 39 c2 75 c8 c3 cc cc cc cc <0f> 0b 90 48 8b 87 b0 00 00 00 48 81 c7 b0 00 00 00 45 31 c0 48 39
[   61.522226] RSP: 0018:ffffbc9800cc36f8 EFLAGS: 00010246
[   61.522557] RAX: 0000000000000001 RBX: 0000000000000300 RCX: 0000000000000001
[   61.523004] RDX: 0000000000000010 RSI: ffffffff8b0de852 RDI: ffff9f03848e5000
[   61.523452] RBP: 0000000000000000 R08: 0000000000000800 R09: 0000000000000000
[   61.523899] R10: ffff9f0384a96800 R11: ffffffffffa48061 R12: ffff9f03849c3000
[   61.524345] R13: 0000000000000300 R14: ffff9f03848e5000 R15: 0000001000000100
[   61.524792] FS:  00007f58cb64d2c0(0000) GS:ffff9f03bbc00000(0000) knlGS:0000000000000000
[   61.525301] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   61.525673] CR2: 00007f6cc629b498 CR3: 000000010498c000 CR4: 00000000000006f0
[   61.526121] Call Trace:
[   61.526284]  <TASK>
[   61.526425]  __veth_napi_enable_range+0xd6/0x230
[   61.526723]  veth_enable_xdp+0xd0/0x160
[   61.526969]  veth_open+0x2e/0xc0
[   61.527180]  __dev_open+0xe2/0x1b0
[   61.527405]  __dev_change_flags+0x1a1/0x210
[   61.527673]  dev_change_flags+0x1c/0x60

This happens because we are calling veth_napi_enable() on already enabled
queues. The root cause is in commit 2e0de636 changed the control logic
dropping this case,

        if (priv->_xdp_prog) {
                err = veth_enable_xdp(dev);
                if (err)
                        return err;
-       } else if (veth_gro_requested(dev)) {
+               /* refer to the logic in veth_xdp_set() */
+               if (!rtnl_dereference(peer_rq->napi)) {
+                       err = veth_napi_enable(peer);
+                       if (err)
+                               return err;
+               }

so that now veth_napi_enable is called if the peer has not yet
initialiazed its peer_rq->napi. The issue is this will happen
even if the NIC is not up. Then in veth_enable_xdp just above
we have similar path,

  veth_enable_xdp
   napi_already_on = (dev->flags & IFF_UP) && rcu_access_pointer(rq->napi)
    err = veth_enable_xdp_range(dev, 0, dev->real_num_rx_queues, napi_already_on);

The trouble is an xdp prog is assigned before bringing the device up each
of the veth_open path will enable the peers xdp napi structs. But then when
we bring the peer up it will similar try to enable again because from
veth_open the IFF_UP flag is not set until after the op in __dev_open so
we believe napi_alread_on = false.

To fix this just drop the IFF_UP test and rely on checking if the napi
struct is enabled. This also matches the peer check in veth_xdp for
disabling.

To reproduce run ./test_xdp_meta.sh I found adding Cilium/Tetragon tests
for XDP.

Fixes: 2e0de636 ("veth: Avoid drop packets when xdp_redirect performs")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20221108221650.808950-2-john.fastabend@gmail.comAcked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

drivers/net/veth.c

@@ -1125,7 +1125,7 @@ static int veth_enable_xdp(struct net_device *dev)
 	int err, i;
 
 	rq = &priv->rq[0];
-	napi_already_on = (dev->flags & IFF_UP) && rcu_access_pointer(rq->napi);
+	napi_already_on = rcu_access_pointer(rq->napi);
 
 	if (!xdp_rxq_info_is_reg(&priv->rq[0].xdp_rxq)) {
 		err = veth_enable_xdp_range(dev, 0, dev->real_num_rx_queues, napi_already_on);