drm/virtio: fix NULL pointer dereference on fence allocation failure

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

drm/virtio: fix NULL pointer dereference on fence allocation failure

virtio_gpu_fence_alloc() can fail due to memory pressure and return NULL,
but its caller like virtio_gpu_init_submit() never checks it. Later,
virtio_gpu_init_submit() passes the NULL fence to
virtio_gpu_fence_event_create(), which unconditionally dereferences it.

Found when fuzzing the virtio driver with Syzkaller:

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000012: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000090-0x0000000000000097]
CPU: 1 UID: 0 PID: 9991 Comm: syz.0.121 Not tainted 7.2.0 #4 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
RIP: 0010:virtio_gpu_fence_event_create drivers/gpu/drm/virtio/virtgpu_submit.c:295 [inline]
RIP: 0010:virtio_gpu_init_submit drivers/gpu/drm/virtio/virtgpu_submit.c:398 [inline]
RIP: 0010:virtio_gpu_execbuffer_ioctl+0xc78/0x1aa0 drivers/gpu/drm/virtio/virtgpu_submit.c:505
Code: 85 ed 0f 85 21 09 00 00 e8 05 5a c9 fb 48 8b 44 24 10 48 8d b8 90 00 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 9a 0d 00 00 48 8b 44 24 10 4c 89 b0 90 00 00 00
RSP: 0018:ffffc900039dfad0 EFLAGS: 00010216
RAX: dffffc0000000000 RBX: ffffc900039dfdd8 RCX: ffffffff85f6fd3d
RDX: 0000000000000012 RSI: ffffffff85f6fd4b RDI: 0000000000000090
RBP: 0000000000000000 R0virtio_gpu_virgl_process_cmd: ctrl 0x102, error 0x1203
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880132c4000
R13: 0000000000000000 R14: ffff888073b6c700 R15: 000000000000003b
FS: 00007fab480b96c0(0000) GS:ffff8880eb6e9000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007effbf5e55a8 CR3: 0000000048d19000 CR4: 0000000000350ef0
Call Trace:
<TASK>
drm_ioctl_kernel+0x1f4/0x3e0 drivers/gpu/drm/drm_ioctl.c:817
drm_ioctl+0x5f4/0xc70 drivers/gpu/drm/drm_ioctl.c:914
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x116/0x800 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fab471a82bd
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fab480b9018 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fab47435fa0 RCX: 00007fab471a82bd
RDX: 00002000000000c0 RSI: 00000000c0406442 RDI: 0000000000000003
RBP: 00007fab480b9080 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fab47436038 R14: 00007fab47435fa0 R15: 00007ffe85ab0740
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:virtio_gpu_fence_event_create drivers/gpu/drm/virtio/virtgpu_submit.c:295 [inline]
RIP: 0010:virtio_gpu_init_submit drivers/gpu/drm/virtio/virtgpu_submit.c:398 [inline]
RIP: 0010:virtio_gpu_execbuffer_ioctl+0xc78/0x1aa0 drivers/gpu/drm/virtio/virtgpu_submit.c:505
Code: 85 ed 0f 85 21 09 00 00 e8 05 5a c9 fb 48 8b 44 24 10 48 8d b8 90 00 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 9a 0d 00 00 48 8b 44 24 10 4c 89 b0 90 00 00 00
RSP: 0018:ffffc900039dfad0 EFLAGS: 00010216
RAX: dffffc0000000000 RBX: ffffc900039dfdd8 RCX: ffffffff85f6fd3d
RDX: 0000000000000012 RSI: ffffffff85f6fd4b RDI: 0000000000000090
RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880132c4000
R13: 0000000000000000 R14: ffff888073b6c700 R15: 000000000000003b
FS: 00007fab480b96c0(0000) GS:ffff888098ae9000(0000) knlGS:00000000
---truncated---