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Date:   Fri, 14 Feb 2020 10:27:12 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v2] KVM: X86: Grab KVM's srcu lock when accessing hv
 assist page

On 14/02/20 10:16, Wanpeng Li wrote:
> From: wanpeng li <wanpengli@...cent.com>
> 
> For the duration of mapping eVMCS, it derefences ->memslots without holding
> ->srcu or ->slots_lock when accessing hv assist page. This patch fixes it by
> moving nested_sync_vmcs12_to_shadow to prepare_guest_switch, where the SRCU
> is already taken.

Looks good, but I'd like an extra review from Sean or Vitaly.

We also should add a WARN_ON_ONCE that replaces the previous location of
the "if (vmx->nested.need_vmcs12_to_shadow_sync)", but I can do that myself.

Thanks,

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@...cent.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 9a66648..6bd6ca4 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1214,6 +1214,9 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
> 
>      vmx_set_host_fs_gs(host_state, fs_sel, gs_sel, fs_base, gs_base);
>      vmx->guest_state_loaded = true;
> +
> +    if (vmx->nested.need_vmcs12_to_shadow_sync)
> +        nested_sync_vmcs12_to_shadow(vcpu);
>  }
> 
>  static void vmx_prepare_switch_to_host(struct vcpu_vmx *vmx)
> @@ -6480,9 +6483,6 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>          vmcs_write32(PLE_WINDOW, vmx->ple_window);
>      }
> 
> -    if (vmx->nested.need_vmcs12_to_shadow_sync)
> -        nested_sync_vmcs12_to_shadow(vcpu);
> -
>      if (kvm_register_is_dirty(vcpu, VCPU_REGS_RSP))
>          vmcs_writel(GUEST_RSP, vcpu->arch.regs[VCPU_REGS_RSP]);
>      if (kvm_register_is_dirty(vcpu, VCPU_REGS_RIP))
> --
> 2.7.4
> 

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