Affected: Linux

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

xen/netfront: destroy queues before real_num_tx_queues is zeroed

xennet_destroy_queues() relies on info->netdev->real_num_tx_queues to delete queues. Since d7dac083414eb5bb99a6d2ed53dc2c1b405224e5 ("net-sysfs: update the queue counts in the unregistration path"), unregister_netdev() indirectly sets real_num_tx_queues to 0. Those two facts together means, that xennet_destroy_queues() called from xennet_remove() 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: 000000000000000…