Re: [PATCH 3/4] KVM: x86: Pend KVM_REQ_APICV_UPDATE during vCPU creation to fix a race

Maxim Levitsky
2022-04-18T13:36:36Z

On Sat, 2022-04-16 at 03:42 +0000, Sean Christopherson wrote:
> Make a KVM_REQ_APICV_UPDATE request when creating a vCPU with an
> in-kernel local APIC and APICv enabled at the module level. Consuming
> kvm_apicv_activated() and stuffing vcpu->arch.apicv_active directly can
> race with __kvm_set_or_clear_apicv_inhibit(), as vCPU creation happens
> before the vCPU is fully onlined, i.e. it won't get the request made to
> "all" vCPUs. If APICv is globally inhibited between setting apicv_active
> and onlining the vCPU, the vCPU will end up running with APICv enabled
> and trigger KVM's sanity check.
>
> Mark APICv as active during vCPU creation if APICv is enabled at the
> module level, both to be optimistic about it's final state, e.g. to avoid
> additional VMWRITEs on VMX, and because there are likely bugs lurking
> since KVM checks apicv_active in multiple vCPU creation paths. While
> keeping the current behavior of consuming kvm_apicv_activated() is
> arguably safer from a regression perspective, force apicv_active so that
> vCPU creation runs with deterministic state and so that if there are bugs,
> they are found sooner than later, i.e. not when some crazy race condition
> is hit.
>
> WARNING: CPU: 0 PID: 484 at arch/x86/kvm/x86.c:9877 vcpu_enter_guest+0x2ae3/0x3ee0 arch/x86/kvm/x86.c:9877

I told you that this warning catches bugs. I am not disappointed!

> Modules linked in:
> CPU: 0 PID: 484 Comm: syz-executor361 Not tainted 5.16.13 #2
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1~cloud0 04/01/2014
> RIP: 0010:vcpu_enter_guest+0x2ae3/0x3ee0 arch/x86/kvm/x86.c:9877
> Call Trace:
> <TASK>
> vcpu_run arch/x86/kvm/x86.c:10039 [inline]
> kvm_arch_vcpu_ioctl_run+0x337/0x15e0 arch/x86/kvm/x86.c:10234
> kvm_vcpu_ioctl+0x4d2/0xc80 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3727
> vfs_ioctl fs/ioctl.c:51 [inline]
> __do_sys_ioctl fs/ioctl.c:874 [inline]
> __se_sys_ioctl fs/ioctl.c:860 [inline]
> __x64_sys_ioctl+0x16d/0x1d0 fs/ioctl.c:860
> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> do_syscall_64+0x38/0x90 arch/x86/entry/common.c:80
> entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> The bug was hit by a syzkaller spamming VM creation with 2 vCPUs and a
> call to KVM_SET_GUEST_DEBUG.
>
> r0 = openat$kvm(0xffffffffffffff9c, &(0x7f0000000000), 0x0, 0x0)
> r1 = ioctl$KVM_CREATE_VM(r0, 0xae01, 0x0)
> ioctl$KVM_CAP_SPLIT_IRQCHIP(r1, 0x4068aea3, &(0x7f0000000000)) (async)
> r2 = ioctl$KVM_CREATE_VCPU(r1, 0xae41, 0x0) (async)
> r3 = ioctl$KVM_CREATE_VCPU(r1, 0xae41, 0x400000000000002)
> ioctl$KVM_SET_GUEST_DEBUG(r3, 0x4048ae9b, &(0x7f00000000c0)={0x5dda9c14aa95f5c5})
> ioctl$KVM_RUN(r2, 0xae80, 0x0)
>
> Reported-by: Gaoning Pan <pgn@zju.edu.cn>
> Reported-by: Yongkang Jia <kangel@zju.edu.cn>
> Fixes: 8df14af42f00 ("kvm: x86: Add support for dynamic APICv activation")
> Cc: stable@vger.kernel.org
> Cc: Maxim Levitsky <mlevitsk@redhat.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/x86.c | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 753296902535..09a270cc1c8f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11259,8 +11259,21 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> r = kvm_create_lapic(vcpu, lapic_timer_advance_ns);
> if (r < 0)
> goto fail_mmu_destroy;
> - if (kvm_apicv_activated(vcpu->kvm))
> +
> + /*
> + * Defer evaluating inhibits until the vCPU is first run, as
> + * this vCPU will not get notified of any changes until this
> + * vCPU is visible to other vCPUs (marked online and added to
> + * the set of vCPUs). Opportunistically mark APICv active as
> + * VMX in particularly is highly unlikely to have inhibits.
> + * Ignore the current per-VM APICv state so that vCPU creation
> + * is guaranteed to run with a deterministic value, the request
> + * will ensure the vCPU gets the correct state before VM-Entry.
> + */
> + if (enable_apicv) {
> vcpu->arch.apicv_active = true;
> + kvm_make_request(KVM_REQ_APICV_UPDATE, vcpu);
> + }
> } else
> static_branch_inc(&kvm_has_noapic_vcpu);
>

Makes sense.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
Maxim Levitsky