From: Paolo Bonzini <pbonzini@redhat.com>
Date: Mon, 4 Nov 2019 13:23:53 +0100
Subject: KVM: fix placement of refcount initialization
Patch-mainline: v5.4-rc8
Git-commit: e2d3fcaf939dded3da604a25ebbea9fb954c2280
References: bsc#1133021

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 decrease this count.  By moving it earlier, we can push
the decrease to out_err_no_arch_destroy_vm without introducing yet another
error label.

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>
Analyzed-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Liang Yan <lyan@suse.com>
---
 virt/kvm/kvm_main.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -668,6 +668,7 @@ static struct kvm *kvm_create_vm(unsigne
 	if (init_srcu_struct(&kvm->irq_srcu))
 		goto out_err_no_irq_srcu;
 
+	refcount_set(&kvm->users_count, 1);
 	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
 		struct kvm_memslots *slots = kvm_alloc_memslots();
 
@@ -685,7 +686,6 @@ static struct kvm *kvm_create_vm(unsigne
 			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;
@@ -723,8 +723,8 @@ out_err_no_mmu_notifier:
 	hardware_disable_all();
 out_err_no_disable:
 	kvm_arch_destroy_vm(kvm);
-	WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
 out_err_no_arch_destroy_vm:
+	WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
 	for (i = 0; i < KVM_NR_BUSES; i++)
 		kfree(kvm_get_bus(kvm, i));
 	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)