linux-kernel - Re: [PATCH] KVM: x86: hyper-v: Task srcu lock when accessing kvm

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Date:   Mon, 10 May 2021 10:48:42 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: hyper-v: Task srcu lock when accessing
 kvm_memslots()

Wanpeng Li <kernellwp@...il.com> writes:

> From: Wanpeng Li <wanpengli@...cent.com>
>
>  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@...cent.com>
> ---
>  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()?

-- 
Vitaly