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Subject: Re: [PATCH RESEND] KVM: X86: Grab KVM's srcu lock when accessing hv
 assist page
To:     Wanpeng Li <kernellwp@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
References: <CANRm+CwmVnJqCzN1sWhBOKZBCqpL2ZfRbT-V+tHMGFwPjCZGvw@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <353a53a7-5d1e-1797-c870-1eb8b382bedd@redhat.com>
Date:   Fri, 14 Feb 2020 09:58:39 +0100
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On 14/02/20 09:51, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> Acquire kvm->srcu for the duration of mapping eVMCS to fix a bug where accessing
> hv assist page derefences ->memslots without holding ->srcu or ->slots_lock.

Perhaps nested_sync_vmcs12_to_shadow should be moved to
prepare_guest_switch, where the SRCU is already taken.

Paolo

> It can be reproduced by running KVM's evmcs_test selftest.
> 
>   =============================
>   WARNING: suspicious RCU usage
>   5.6.0-rc1+ #53 Tainted: G        W IOE
>   -----------------------------
>   ./include/linux/kvm_host.h:623 suspicious rcu_dereference_check() usage!
> 
>   other info that might help us debug this:
> 
>    rcu_scheduler_active = 2, debug_locks = 1
>   1 lock held by evmcs_test/8507:
>    #0: ffff9ddd156d00d0 (&vcpu->mutex){+.+.}, at:
> kvm_vcpu_ioctl+0x85/0x680 [kvm]
> 
>   stack backtrace:
>   CPU: 6 PID: 8507 Comm: evmcs_test Tainted: G        W IOE     5.6.0-rc1+ #53
>   Hardware name: Dell Inc. OptiPlex 7040/0JCTF8, BIOS 1.4.9 09/12/2016
>   Call Trace:
>    dump_stack+0x68/0x9b
>    kvm_read_guest_cached+0x11d/0x150 [kvm]
>    kvm_hv_get_assist_page+0x33/0x40 [kvm]
>    nested_enlightened_vmentry+0x2c/0x60 [kvm_intel]
>    nested_vmx_handle_enlightened_vmptrld.part.52+0x32/0x1c0 [kvm_intel]
>    nested_sync_vmcs12_to_shadow+0x439/0x680 [kvm_intel]
>    vmx_vcpu_run+0x67a/0xe60 [kvm_intel]
>    vcpu_enter_guest+0x35e/0x1bc0 [kvm]
>    kvm_arch_vcpu_ioctl_run+0x40b/0x670 [kvm]
>    kvm_vcpu_ioctl+0x370/0x680 [kvm]
>    ksys_ioctl+0x235/0x850
>    __x64_sys_ioctl+0x16/0x20
>    do_syscall_64+0x77/0x780
>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 657c2ed..a68a69d 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -1994,14 +1994,18 @@ static int
> nested_vmx_handle_enlightened_vmptrld(struct kvm_vcpu *vcpu,
>  void nested_sync_vmcs12_to_shadow(struct kvm_vcpu *vcpu)
>  {
>      struct vcpu_vmx *vmx = to_vmx(vcpu);
> +    int idx;
> 
>      /*
>       * hv_evmcs may end up being not mapped after migration (when
>       * L2 was running), map it here to make sure vmcs12 changes are
>       * properly reflected.
>       */
> -    if (vmx->nested.enlightened_vmcs_enabled && !vmx->nested.hv_evmcs)
> +    if (vmx->nested.enlightened_vmcs_enabled && !vmx->nested.hv_evmcs) {
> +        idx = srcu_read_lock(&vcpu->kvm->srcu);
>          nested_vmx_handle_enlightened_vmptrld(vcpu, false);
> +        srcu_read_unlock(&vcpu->kvm->srcu, idx);
> +    }
> 
>      if (vmx->nested.hv_evmcs) {
>          copy_vmcs12_to_enlightened(vmx);
> --
> 2.7.4
>