vhost: fix hung thread due to erroneous iotlb entries

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

vhost: fix hung thread due to erroneous iotlb entries

In vhost_iotlb_add_range_ctx(), range size can overflow to 0 when

start is 0 and last is ULONG_MAX. One instance where it can happen

is when userspace sends an IOTLB message with iova=size=uaddr=0

(vhost_process_iotlb_msg). So, an entry with size = 0, start = 0,

last = ULONG_MAX ends up in the iotlb. Next time a packet is sent,

iotlb_access_ok() loops indefinitely due to that erroneous entry.

Call Trace:

<TASK>

iotlb_access_ok+0x21b/0x3e0 drivers/vhost/vhost.c:1340

vq_meta_prefetch+0xbc/0x280 drivers/vhost/vhost.c:1366

vhost_transport_do_send_pkt+0xe0/0xfd0 drivers/vhost/vsock.c:104

vhost_worker+0x23d/0x3d0 drivers/vhost/vhost.c:372

kthread+0x2e9/0x3a0 kernel/kthread.c:377

ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

</TASK>

Reported by syzbot at:

https://syzkaller.appspot.com/bug?extid=0abd373e2e50d704db87

To fix this, do two things:

1. Return -EINVAL in vhost_chr_write_iter() when userspace asks to map

a range with size 0.

2. Fix vhost_iotlb_add_range_ctx() to handle the range [0, ULONG_MAX]

by splitting it into two entries.