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Subject: Re: [PATCH v2] KVM: nVMX: do not use dangling shadow VMCS after guest
 reset
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <a79fe081-d46a-07a1-7453-2250fac37374@redhat.com>
Date:   Sat, 20 Jul 2019 01:23:07 +0300
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        stable@vger.kernel.org
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Message-Id: <9422299F-A088-4FCD-9B0B-80F7B3F9615E@oracle.com>
References: <1563572390-28823-1-git-send-email-pbonzini@redhat.com>
 <A523C8F1-2A15-4B14-AB83-9D2659A7E78F@oracle.com>
 <a79fe081-d46a-07a1-7453-2250fac37374@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On 20 Jul 2019, at 1:21, Paolo Bonzini <pbonzini@redhat.com> wrote:
>=20
> On 20/07/19 00:06, Liran Alon wrote:
>>=20
>>=20
>>> On 20 Jul 2019, at 0:39, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>>=20
>>> If a KVM guest is reset while running a nested guest, free_nested =
will
>>> disable the shadow VMCS execution control in the vmcs01.  However,
>>> on the next KVM_RUN vmx_vcpu_run would nevertheless try to sync
>>> the VMCS12 to the shadow VMCS which has since been freed.
>>>=20
>>> This causes a vmptrld of a NULL pointer on my machime, but Jan =
reports
>>> the host to hang altogether.  Let's see how much this trivial patch =
fixes.
>>>=20
>>> Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
>>> Cc: Liran Alon <liran.alon@oracle.com>
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>=20
>> 1) Are we sure we prefer WARN_ON() instead of WARN_ON_ONCE()?
>=20
> I don't think you can get it to be called in a loop, the calls are
> generally guarded by ifs.
>=20
>> 2) Should we also check for WARN_ON(!vmcs12)? As free_nested() also =
kfree(vmx->nested.cached_vmcs12).
>=20
> Well, it doesn't NULL it but it does NULL shadow_vmcs so the extra
> warning wouldn't add much.
>=20
>> In fact, because free_nested() don=E2=80=99t put NULL in =
cached_vmcs12 after kfree() it, I wonder if we shouldn=E2=80=99t create =
a separate patch that does:
>> (a) Modify free_nested() to put NULL in cached_vmcs12 after kfree().
>> (b) Put BUG_ON(!cached_vmcs12) in get_vmcs12() before returning =
value.
>=20
> This is useful but a separate improvement (and not a bugfix, I want =
this
> patch to be small so it applies to older trees).
>=20
> Paolo

ACK on all the above. :)
Reviewed-by:  Liran Alon <liran.alon@oracle.com>

-Liran

>=20
>> -Liran
>>=20
>>> ---
>>> arch/x86/kvm/vmx/nested.c | 8 +++++++-
>>> 1 file changed, 7 insertions(+), 1 deletion(-)
>>>=20
>>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>>> index 4f23e34f628b..0f1378789bd0 100644
>>> --- a/arch/x86/kvm/vmx/nested.c
>>> +++ b/arch/x86/kvm/vmx/nested.c
>>> @@ -194,6 +194,7 @@ static void vmx_disable_shadow_vmcs(struct =
vcpu_vmx *vmx)
>>> {
>>> 	secondary_exec_controls_clearbit(vmx, =
SECONDARY_EXEC_SHADOW_VMCS);
>>> 	vmcs_write64(VMCS_LINK_POINTER, -1ull);
>>> +	vmx->nested.need_vmcs12_to_shadow_sync =3D false;
>>> }
>>>=20
>>> static inline void nested_release_evmcs(struct kvm_vcpu *vcpu)
>>> @@ -1341,6 +1342,9 @@ static void copy_shadow_to_vmcs12(struct =
vcpu_vmx *vmx)
>>> 	unsigned long val;
>>> 	int i;
>>>=20
>>> +	if (WARN_ON(!shadow_vmcs))
>>> +		return;
>>> +
>>> 	preempt_disable();
>>>=20
>>> 	vmcs_load(shadow_vmcs);
>>> @@ -1373,6 +1377,9 @@ static void copy_vmcs12_to_shadow(struct =
vcpu_vmx *vmx)
>>> 	unsigned long val;
>>> 	int i, q;
>>>=20
>>> +	if (WARN_ON(!shadow_vmcs))
>>> +		return;
>>> +
>>> 	vmcs_load(shadow_vmcs);
>>>=20
>>> 	for (q =3D 0; q < ARRAY_SIZE(fields); q++) {
>>> @@ -4436,7 +4443,6 @@ static inline void =
nested_release_vmcs12(struct kvm_vcpu *vcpu)
>>> 		/* copy to memory all shadowed fields in case
>>> 		   they were modified */
>>> 		copy_shadow_to_vmcs12(vmx);
>>> -		vmx->nested.need_vmcs12_to_shadow_sync =3D false;
>>> 		vmx_disable_shadow_vmcs(vmx);
>>> 	}
>>> 	vmx->nested.posted_intr_nv =3D -1;
>>> --=20
>>> 1.8.3.1
>>>=20
>>=20
>=20