Re: [PATCH] KVM: x86: hyper-v: Task srcu lock when accessing kvm_memslots()

[Wanpeng Li]

On Mon, 10 May 2021 at 16:48, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> Wanpeng Li <kernellwp@xxxxxxxxx> writes:
>
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> >  WARNING: suspicious RCU usage
> >  5.13.0-rc1 #4 Not tainted
> >  -----------------------------
> >  ./include/linux/kvm_host.h:710 suspicious rcu_dereference_check() usage!
> >
> > other info that might help us debug this:
> >
> > rcu_scheduler_active = 2, debug_locks = 1
> >  1 lock held by hyperv_clock/8318:
> >   #0: ffffb6b8cb05a7d8 (&hv->hv_lock){+.+.}-{3:3}, at: kvm_hv_invalidate_tsc_page+0x3e/0xa0 [kvm]
> >
> > stack backtrace:
> > CPU: 3 PID: 8318 Comm: hyperv_clock Not tainted 5.13.0-rc1 #4
> > Call Trace:
> >  dump_stack+0x87/0xb7
> >  lockdep_rcu_suspicious+0xce/0xf0
> >  kvm_write_guest_page+0x1c1/0x1d0 [kvm]
> >  kvm_write_guest+0x50/0x90 [kvm]
> >  kvm_hv_invalidate_tsc_page+0x79/0xa0 [kvm]
> >  kvm_gen_update_masterclock+0x1d/0x110 [kvm]
> >  kvm_arch_vm_ioctl+0x2a7/0xc50 [kvm]
> >  kvm_vm_ioctl+0x123/0x11d0 [kvm]
> >  __x64_sys_ioctl+0x3ed/0x9d0
> >  do_syscall_64+0x3d/0x80
> >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> >
> > kvm_memslots() will be called by kvm_write_guest(), so we should take the srcu lock.
> >
> > Fixes: e880c6ea5 (KVM: x86: hyper-v: Prevent using not-yet-updated TSC page by secondary CPUs)
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/hyperv.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> > index f98370a3..f00830e 100644
> > --- a/arch/x86/kvm/hyperv.c
> > +++ b/arch/x86/kvm/hyperv.c
> > @@ -1172,6 +1172,7 @@ void kvm_hv_invalidate_tsc_page(struct kvm *kvm)
> >  {
> >       struct kvm_hv *hv = to_kvm_hv(kvm);
> >       u64 gfn;
> > +     int idx;
> >
> >       if (hv->hv_tsc_page_status == HV_TSC_PAGE_BROKEN ||
> >           hv->hv_tsc_page_status == HV_TSC_PAGE_UNSET ||
> > @@ -1190,9 +1191,16 @@ void kvm_hv_invalidate_tsc_page(struct kvm *kvm)
> >       gfn = hv->hv_tsc_page >> HV_X64_MSR_TSC_REFERENCE_ADDRESS_SHIFT;
> >
> >       hv->tsc_ref.tsc_sequence = 0;
> > +
> > +     /*
> > +      * Take the srcu lock as memslots will be accessed to check the gfn
> > +      * cache generation against the memslots generation.
> > +      */
> > +     idx = srcu_read_lock(&kvm->srcu);
> >       if (kvm_write_guest(kvm, gfn_to_gpa(gfn),
> >                           &hv->tsc_ref, sizeof(hv->tsc_ref.tsc_sequence)))
> >               hv->hv_tsc_page_status = HV_TSC_PAGE_BROKEN;
> > +     srcu_read_unlock(&kvm->srcu, idx);
> >
> >  out_unlock:
> >       mutex_unlock(&hv->hv_lock);
>
> Thanks!
>
> Do we need to do the same in kvm_hv_setup_tsc_page()?

kvm_hv_setup_tsc_page() is called in vcpu_enter_guest() path which has
already held kvm->srcu lock.

    Wanpeng