[PATCH] virtio_mem: fix panic on mb_states indexing overflow

[PATCH] virtio_mem: fix panic on mb_states indexing overflow

[Eric Ren]

`mb_id` is unsigned integer, which is used to index
`mb_states` array in reverse order. `mb_id` can decrease
to `0UL - 1` that is a very large number, causing invalid
address access.

The calltrace is like below:
```
[  286.344977] BUG: unable to handle page fault for address: ffffa95180cf8fff
[  286.345800] #PF: supervisor read access in kernel mode
[  286.346738] #PF: error_code(0x0000) - not-present page
[  286.347440] PGD 1000067 P4D 1000067 PUD 138c067 PMD 1840435067 PTE 0
[  286.348156] Oops: 0000 [#1] SMP PTI                                                        [  286.348556] CPU: 1 PID: 122 Comm: kworker/1:2 Tainted: G           OE
...
[  286.350740] Workqueue: events_freezable virtio_mem_run_wq [virtio_mem]
[  286.351605] RIP: 0010:virtio_mem_unplug_request+0x418/0x890 [virtio_mem]
[  286.352519] Code: 0f 87 fc 00 00 00 4a 63 54 ac 30 48 83 bc d5 f8 00 00 00 00 48 89 d0 0f 8
4 e5 00 00 00 48 8b b5 38 01 00 00 4c 89 e2 48 29 ca <0f> b6 34 16 39 c6 75 c7 40 80 fe 02 0f
82 a4 01 00 00 40 80 fe 03
[  286.355030] RSP: 0018:ffffa95181c4bd50 EFLAGS: 00010286
[  286.355737] RAX: 0000000000000005 RBX: 0000000000006100 RCX: 0000000000000000
[  286.356752] RDX: ffffffffffffffff RSI: ffffa95180cf9000 RDI: ffff8e5dc393b348
[  286.357649] RBP: ffff8e5dc393b200 R08: ffff8e463cd2b610 R09: 0000000000000021
[  286.358627] R10: ffffa95181c4bcd0 R11: ffffa95181c4baa0 R12: ffffffffffffffff
[  286.359617] R13: 0000000000000003 R14: ffff8e5dc393b348 R15: 00000000fffffff0
[  286.360503] FS:  0000000000000000(0000) GS:ffff8e463cd00000(0000) knlGS:0000000000000000
[  286.361532] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  286.362229] CR2: ffffa95180cf8fff CR3: 0000001846234000 CR4: 00000000000006e0
[  286.363168] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  286.364162] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  286.365054] Call Trace:
[  286.365431]  ? virtio_mem_run_wq+0x5a4/0x870 [virtio_mem]
[  286.366132]  ? __schedule+0x4b3/0x800
[  286.366547]  ? process_one_work+0x18b/0x350
[  286.367041]  ? worker_thread+0x4f/0x3a0
[  286.367675]  ? rescuer_thread+0x350/0x350
[  286.368234]  ? kthread+0xfa/0x130
[  286.368605]  ? kthread_create_worker_on_cpu+0x70/0x70
[  286.369155]  ? ret_from_fork+0x1f/0x30
```

Fixes by also checking its up boundary.

Signed-off-by: Eric Ren <renzhengeek@gmail.com>
Signed-off-by: Eric Ren <renzhen.rz@alibaba-inc.com>
---
 drivers/virtio/virtio_mem.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
index 96e5a8782769..e403e6d95c0b 100644
--- a/drivers/virtio/virtio_mem.c
+++ b/drivers/virtio/virtio_mem.c
@@ -486,7 +486,9 @@ static int virtio_mem_sbm_mb_states_prepare_next_mb(struct virtio_mem *vm)
 
 #define virtio_mem_sbm_for_each_mb_rev(_vm, _mb_id, _state) \
 	for (_mb_id = _vm->sbm.next_mb_id - 1; \
-	     _mb_id >= _vm->sbm.first_mb_id && _vm->sbm.mb_count[_state]; \
+	     _mb_id >= _vm->sbm.first_mb_id && \
+	     _mb_id < vm->sbm.next_mb_id &&		\
+	     _vm->sbm.mb_count[_state]; \
 	     _mb_id--) \
 		if (virtio_mem_sbm_get_mb_state(_vm, _mb_id) == _state)
 
-- 
2.30.1 (Apple Git-130)