vhost/scsi: null-ptr-dereference in vhost_scsi_get_req()

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

vhost/scsi: null-ptr-dereference in vhost_scsi_get_req()

Since commit 3f8ca2e115e5 ("vhost/scsi: Extract common handling code
from control queue handler") a null pointer dereference bug can be
triggered when guest sends an SCSI AN request.

In vhost_scsi_ctl_handle_vq(), `vc.target` is assigned with
`&v_req.tmf.lun[1]` within a switch-case block and is then passed to
vhost_scsi_get_req() which extracts `vc->req` and `tpg`. However, for
a `VIRTIO_SCSI_T_AN_*` request, tpg is not required, so `vc.target` is
set to NULL in this branch. Later, in vhost_scsi_get_req(),
`vc->target` is dereferenced without being checked, leading to a null
pointer dereference bug. This bug can be triggered from guest.

When this bug occurs, the vhost_worker process is killed while holding
`vq->mutex` and the corresponding tpg will remain occupied
indefinitely.

Below is the KASAN report:
Oops: general protection fault, probably for non-canonical address
0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 1 PID: 840 Comm: poc Not tainted 6.10.0+ #1
Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS
1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:vhost_scsi_get_req+0x165/0x3a0
Code: 00 fc ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 2b 02 00 00
48 b8 00 00 00 00 00 fc ff df 4d 8b 65 30 4c 89 e2 48 c1 ea 03 <0f> b6
04 02 4c 89 e2 83 e2 07 38 d0 7f 08 84 c0 0f 85 be 01 00 00
RSP: 0018:ffff888017affb50 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff88801b000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff888017affcb8
RBP: ffff888017affb80 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: ffff888017affc88 R14: ffff888017affd1c R15: ffff888017993000
FS: 000055556e076500(0000) GS:ffff88806b100000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000200027c0 CR3: 0000000010ed0004 CR4: 0000000000370ef0
Call Trace:
<TASK>
? show_regs+0x86/0xa0
? die_addr+0x4b/0xd0
? exc_general_protection+0x163/0x260
? asm_exc_general_protection+0x27/0x30
? vhost_scsi_get_req+0x165/0x3a0
vhost_scsi_ctl_handle_vq+0x2a4/0xca0
? __pfx_vhost_scsi_ctl_handle_vq+0x10/0x10
? __switch_to+0x721/0xeb0
? __schedule+0xda5/0x5710
? __kasan_check_write+0x14/0x30
? _raw_spin_lock+0x82/0xf0
vhost_scsi_ctl_handle_kick+0x52/0x90
vhost_run_work_list+0x134/0x1b0
vhost_task_fn+0x121/0x350
...
</TASK>
---[ end trace 0000000000000000 ]---

Let's add a check in vhost_scsi_get_req.

[whitespace fixes]