Re: [PATCH] xen-blkfront: Handle NULL gendisk

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

On Wed, Jun 01, 2022 at 03:53:41PM -0400, Jason Andryuk wrote:
> 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: 05d69d950d9d "xen-blkfront: sanitize the removal state machine"
> Reported-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>

Tis looks ok, but do we have anything that prevents races between
blkfront_connect, blkfront_closing and blkfront_remove?