When a VBD is not fully created and then closed, the kernel can have a
NULL pointer dereference:

The reproducer is trivial:

[user@dom0 ~]$ sudo xl block-attach work backend=sys-usb vdev=xvdi target=/dev/sdz
[user@dom0 ~]$ xl block-list work
Vdev  BE  handle state evt-ch ring-ref BE-path
51712 0   241    4     -1     -1       /local/domain/0/backend/vbd/241/51712
51728 0   241    4     -1     -1       /local/domain/0/backend/vbd/241/51728
51744 0   241    4     -1     -1       /local/domain/0/backend/vbd/241/51744
51760 0   241    4     -1     -1       /local/domain/0/backend/vbd/241/51760
51840 3   241    3     -1     -1       /local/domain/3/backend/vbd/241/51840
                 ^ note state, the /dev/sdz doesn't exist in the backend

[user@dom0 ~]$ sudo xl block-detach work xvdi
[user@dom0 ~]$ xl block-list work
Vdev  BE  handle state evt-ch ring-ref BE-path
work is an invalid domain identifier

And its console has:

BUG: kernel NULL pointer dereference, address: 0000000000000050
PGD 80000000edebb067 P4D 80000000edebb067 PUD edec2067 PMD 0
Oops: 0000 [#1] PREEMPT SMP PTI
CPU: 1 PID: 52 Comm: xenwatch Not tainted 5.16.18-2.43.fc32.qubes.x86_64 #1
RIP: 0010:blk_mq_stop_hw_queues+0x5/0x40
Code: 00 48 83 e0 fd 83 c3 01 48 89 85 a8 00 00 00 41 39 5c 24 50 77 c0 5b 5d 41 5c 41 5d c3 c3 0f 1f 80 00 00 00 00 0f 1f 44 00 00 <8b> 47 50 85 c0 74 32 41 54 49 89 fc 55 53 31 db 49 8b 44 24 48 48
RSP: 0018:ffffc90000bcfe98 EFLAGS: 00010293
RAX: ffffffffc0008370 RBX: 0000000000000005 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000005 RDI: 0000000000000000
RBP: ffff88800775f000 R08: 0000000000000001 R09: ffff888006e620b8
R10: ffff888006e620b0 R11: f000000000000000 R12: ffff8880bff39000
R13: ffff8880bff39000 R14: 0000000000000000 R15: ffff88800604be00
FS:  0000000000000000(0000) GS:ffff8880f3300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000050 CR3: 00000000e932e002 CR4: 00000000003706e0
Call Trace:
 <TASK>
 blkback_changed+0x95/0x137 [xen_blkfront]
 ? read_reply+0x160/0x160
 xenwatch_thread+0xc0/0x1a0
 ? do_wait_intr_irq+0xa0/0xa0
 kthread+0x16b/0x190
 ? set_kthread_struct+0x40/0x40
 ret_from_fork+0x22/0x30
 </TASK>
Modules linked in: snd_seq_dummy snd_hrtimer snd_seq snd_seq_device snd_timer snd soundcore ipt_REJECT nf_reject_ipv4 xt_state xt_conntrack nft_counter nft_chain_nat xt_MASQUERADE nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nft_compat nf_tables nfnetlink intel_rapl_msr intel_rapl_common crct10dif_pclmul crc32_pclmul crc32c_intel ghash_clmulni_intel xen_netfront pcspkr xen_scsiback target_core_mod xen_netback xen_privcmd xen_gntdev xen_gntalloc xen_blkback xen_evtchn ipmi_devintf ipmi_msghandler fuse bpf_preload ip_tables overlay xen_blkfront
CR2: 0000000000000050
---[ end trace 7bc9597fd06ae89d ]---
RIP: 0010:blk_mq_stop_hw_queues+0x5/0x40
Code: 00 48 83 e0 fd 83 c3 01 48 89 85 a8 00 00 00 41 39 5c 24 50 77 c0 5b 5d 41 5c 41 5d c3 c3 0f 1f 80 00 00 00 00 0f 1f 44 00 00 <8b> 47 50 85 c0 74 32 41 54 49 89 fc 55 53 31 db 49 8b 44 24 48 48
RSP: 0018:ffffc90000bcfe98 EFLAGS: 00010293
RAX: ffffffffc0008370 RBX: 0000000000000005 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000005 RDI: 0000000000000000
RBP: ffff88800775f000 R08: 0000000000000001 R09: ffff888006e620b8
R10: ffff888006e620b0 R11: f000000000000000 R12: ffff8880bff39000
R13: ffff8880bff39000 R14: 0000000000000000 R15: ffff88800604be00
FS:  0000000000000000(0000) GS:ffff8880f3300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000050 CR3: 00000000e932e002 CR4: 00000000003706e0
Kernel panic - not syncing: Fatal exception
Kernel Offset: disabled

info->rq and info->gd are only set in blkfront_connect(), which is
called for state 4 (XenbusStateConnected).  Guard against using NULL
variables in blkfront_closing() to avoid the issue.

The rest of blkfront_closing looks okay.  If info->nr_rings is 0, then
for_each_rinfo won't do anything.

blkfront_remove also needs to check for non-NULL pointers before
cleaning up the gendisk and request queue.

Fixes: 05d69d95 "xen-blkfront: sanitize the removal state machine"
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220601195341.28581-1-jandryuk@gmail.comSigned-off-by: Juergen Gross <jgross@suse.com>