CVE-2022-48914

In the Linux kernel, the following vulnerability has been resolved:

xen/netfront: destroy queues before realnumtx_queues is zeroed

xennetdestroyqueues() relies on info->netdev->realnumtxqueues to delete queues. Since d7dac083414eb5bb99a6d2ed53dc2c1b405224e5 ("net-sysfs: update the queue counts in the unregistration path"), unregisternetdev() indirectly sets realnumtxqueues to 0. Those two facts together means, that xennetdestroyqueues() called from xennetremove() cannot do its job, because it's called after unregister_netdev(). This results in kfree-ing queues that are still linked in napi, which ultimately crashes:

BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP PTI
CPU: 1 PID: 52 Comm: xenwatch Tainted: G        W         5.16.10-1.32.fc32.qubes.x86_64+ #226
RIP: 0010:free_netdev+0xa3/0x1a0
Code: ff 48 89 df e8 2e e9 00 00 48 8b 43 50 48 8b 08 48 8d b8 a0 fe ff ff 48 8d a9 a0 fe ff ff 49 39 c4 75 26 eb 47 e8 ed c1 66 ff <48> 8b 85 60 01 00 00 48 8d 95 60 01 00 00 48 89 ef 48 2d 60 01 00
RSP: 0000:ffffc90000bcfd00 EFLAGS: 00010286
RAX: 0000000000000000 RBX: ffff88800edad000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ffffc90000bcfc30 RDI: 00000000ffffffff
RBP: fffffffffffffea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000001 R12: ffff88800edad050
R13: ffff8880065f8f88 R14: 0000000000000000 R15: ffff8880066c6680
FS:  0000000000000000(0000) GS:ffff8880f3300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 00000000e998c006 CR4: 00000000003706e0
Call Trace:
 <TASK>
 xennet_remove+0x13d/0x300 [xen_netfront]
 xenbus_dev_remove+0x6d/0xf0
 __device_release_driver+0x17a/0x240
 device_release_driver+0x24/0x30
 bus_remove_device+0xd8/0x140
 device_del+0x18b/0x410
 ? _raw_spin_unlock+0x16/0x30
 ? klist_iter_exit+0x14/0x20
 ? xenbus_dev_request_and_reply+0x80/0x80
 device_unregister+0x13/0x60
 xenbus_dev_changed+0x18e/0x1f0
 xenwatch_thread+0xc0/0x1a0
 ? do_wait_intr_irq+0xa0/0xa0
 kthread+0x16b/0x190
 ? set_kthread_struct+0x40/0x40
 ret_from_fork+0x22/0x30
 </TASK>

Fix this by calling xennetdestroyqueues() from xennetuninit(), when realnumtxqueues is still available. This ensures that queues are destroyed when realnumtxqueues is set to 0, regardless of how unregisternetdev() was called.

Originally reported at https://github.com/QubesOS/qubes-issues/issues/7257