CVE-2022-48943: KVM: x86/mmu: make apf token non-zero to fix bug

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

KVM: x86/mmu: make apf token non-zero to fix bug

In current async pagefault logic, when a page is ready, KVM relies on kvmarchcandequeueasyncpagepresent() to determine whether to deliver a READY event to the Guest. This function test token value of struct kvmvcpupvapfdata, which must be reset to zero by Guest kernel when a READY event is finished by Guest. If value is zero meaning that a READY event is done, so the KVM can deliver another. But the kvmarchsetupasyncpf() may produce a valid token with zero value, which is confused with previous mention and may lead the loss of this READY event.

This bug may cause task blocked forever in Guest: INFO: task stress:7532 blocked for more than 1254 seconds. Not tainted 5.10.0 #16 "echo 0 > /proc/sys/kernel/hungtasktimeoutsecs" disables this message. task:stress state:D stack: 0 pid: 7532 ppid: 1409 flags:0x00000080 Call Trace: schedule+0x1e7/0x650 schedule+0x46/0xb0 kvmasyncpftaskwaitschedule+0xad/0xe0 ? exittousermodeprepare+0x60/0x70 kvmhandleasyncpf+0x4f/0xb0 ? asmexcpagefault+0x8/0x30 excpagefault+0x6f/0x110 ? asmexcpagefault+0x8/0x30 asmexcpagefault+0x1e/0x30 RIP: 0033:0x402d00 RSP: 002b:00007ffd31912500 EFLAGS: 00010206 RAX: 0000000000071000 RBX: ffffffffffffffff RCX: 00000000021a32b0 RDX: 000000000007d011 RSI: 000000000007d000 RDI: 00000000021262b0 RBP: 00000000021262b0 R08: 0000000000000003 R09: 0000000000000086 R10: 00000000000000eb R11: 00007fefbdf2baa0 R12: 0000000000000000 R13: 0000000000000002 R14: 000000000007d000 R15: 0000000000001000