xen/virtio: Fix NULL deref when a bridge of PCI root bus has no parent

xen/virtio: Fix NULL deref when a bridge of PCI root bus has no parent

When attempting to run Xen on a QEMU/KVM virtual machine with virtio
devices (all x86_64), function xen_dt_get_node() crashes on accessing
bus->bridge->parent->of_node because a bridge of the PCI root bus has no
parent set:

[    1.694192][    T1] BUG: kernel NULL pointer dereference, address: 0000000000000288
[    1.695688][    T1] #PF: supervisor read access in kernel mode
[    1.696297][    T1] #PF: error_code(0x0000) - not-present page
[    1.696297][    T1] PGD 0 P4D 0
[    1.696297][    T1] Oops: 0000 [#1] PREEMPT SMP NOPTI
[    1.696297][    T1] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.3.7-1-default #1 openSUSE Tumbleweed a577eae57964bb7e83477b5a5645a1781df990f0
[    1.696297][    T1] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.15.0-0-g2dd4b9b-rebuilt.opensuse.org 04/01/2014
[    1.696297][    T1] RIP: e030:xen_virtio_restricted_mem_acc+0xd9/0x1c0
[    1.696297][    T1] Code: 45 0c 83 e8 c9 a3 ea ff 31 c0 eb d7 48 8b 87 40 ff ff ff 48 89 c2 48 8b 40 10 48 85 c0 75 f4 48 8b 82 10 01 00 00 48 8b 40 40 <48> 83 b8 88 02 00 00 00 0f 84 45 ff ff ff 66 90 31 c0 eb a5 48 89
[    1.696297][    T1] RSP: e02b:ffffc90040013cc8 EFLAGS: 00010246
[    1.696297][    T1] RAX: 0000000000000000 RBX: ffff888006c75000 RCX: 0000000000000029
[    1.696297][    T1] RDX: ffff888005ed1000 RSI: ffffc900400f100c RDI: ffff888005ee30d0
[    1.696297][    T1] RBP: ffff888006c75010 R08: 0000000000000001 R09: 0000000330000006
[    1.696297][    T1] R10: ffff888005850028 R11: 0000000000000002 R12: ffffffff830439a0
[    1.696297][    T1] R13: 0000000000000000 R14: ffff888005657900 R15: ffff888006e3e1e8
[    1.696297][    T1] FS:  0000000000000000(0000) GS:ffff88804a000000(0000) knlGS:0000000000000000
[    1.696297][    T1] CS:  e030 DS: 0000 ES: 0000 CR0: 0000000080050033
[    1.696297][    T1] CR2: 0000000000000288 CR3: 0000000002e36000 CR4: 0000000000050660
[    1.696297][    T1] Call Trace:
[    1.696297][    T1]  <TASK>
[    1.696297][    T1]  virtio_features_ok+0x1b/0xd0
[    1.696297][    T1]  virtio_dev_probe+0x19c/0x270
[    1.696297][    T1]  really_probe+0x19b/0x3e0
[    1.696297][    T1]  __driver_probe_device+0x78/0x160
[    1.696297][    T1]  driver_probe_device+0x1f/0x90
[    1.696297][    T1]  __driver_attach+0xd2/0x1c0
[    1.696297][    T1]  bus_for_each_dev+0x74/0xc0
[    1.696297][    T1]  bus_add_driver+0x116/0x220
[    1.696297][    T1]  driver_register+0x59/0x100
[    1.696297][    T1]  virtio_console_init+0x7f/0x110
[    1.696297][    T1]  do_one_initcall+0x47/0x220
[    1.696297][    T1]  kernel_init_freeable+0x328/0x480
[    1.696297][    T1]  kernel_init+0x1a/0x1c0
[    1.696297][    T1]  ret_from_fork+0x29/0x50
[    1.696297][    T1]  </TASK>
[    1.696297][    T1] Modules linked in:
[    1.696297][    T1] CR2: 0000000000000288
[    1.696297][    T1] ---[ end trace 0000000000000000 ]---

The PCI root bus is in this case created from ACPI description via
acpi_pci_root_add() -> pci_acpi_scan_root() -> acpi_pci_root_create() ->
pci_create_root_bus() where the last function is called with
parent=NULL. It indicates that no parent is present and then
bus->bridge->parent is NULL too.

Fix the problem by checking bus->bridge->parent in xen_dt_get_node() for
NULL first.

Fixes: ef8ae384b4c9 ("xen/virtio: Handle PCI devices which Host controller is described in DT")
Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Link: https://lore.kernel.org/r/20230621131214.9398-2-petr.pavlu@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>