[PATCH 2/2] KVM: Fix rcu splat if vm creation fails

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

Reported by syzkaller:

   =============================
   WARNING: suspicious RCU usage
   -----------------------------
   ./include/linux/kvm_host.h:536 suspicious rcu_dereference_check() usage!
   
   other info that might help us debug this:

   rcu_scheduler_active = 2, debug_locks = 1
   no locks held by repro_11/12688.
    
   stack backtrace:
   Call Trace:
    dump_stack+0x7d/0xc5
    lockdep_rcu_suspicious+0x123/0x170
    kvm_dev_ioctl+0x9a9/0x1260 [kvm]
    do_vfs_ioctl+0x1a1/0xfb0
    ksys_ioctl+0x6d/0x80
    __x64_sys_ioctl+0x73/0xb0
    do_syscall_64+0x108/0xaa0
    entry_SYSCALL_64_after_hwframe+0x49/0xbe

Commit a97b0e773e4 (kvm: call kvm_arch_destroy_vm if vm creation fails)
sets users_count to 1 before kvm_arch_init_vm(), however, if kvm_arch_init_vm()
fails, we need to dec this count. Or, we can move the sets refcount after 
kvm_arch_init_vm().

syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=15209b84e00000

Reported-by: syzbot+75475908cd0910f141ee@syzkaller.appspotmail.com
Fixes: a97b0e773e49 ("kvm: call kvm_arch_destroy_vm if vm creation fails")
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 virt/kvm/kvm_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d6f0696..62ae0c9 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -662,11 +662,11 @@ static struct kvm *kvm_create_vm(unsigned long type)
 			goto out_err_no_arch_destroy_vm;
 	}
 
-	refcount_set(&kvm->users_count, 1);
 	r = kvm_arch_init_vm(kvm, type);
 	if (r)
 		goto out_err_no_arch_destroy_vm;
 
+	refcount_set(&kvm->users_count, 1);
 	r = hardware_enable_all();
 	if (r)
 		goto out_err_no_disable;
-- 
2.7.4