On Mon, 4 Nov 2019 at 19:18, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 04/11/19 07:27, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > 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().
>
> I don't understand this one, hasn't
>
> WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
>
> decreased the conut already? With your patch the refcount would then
> underflow.
r = kvm_arch_init_vm(kvm, type);
if (r)
goto out_err_no_arch_destroy_vm;
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:
So, if kvm_arch_init_vm() fails, we will not execute
WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
Wanpeng
