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Subject: Re: [PATCH] KVM: LAPIC: Fix pv ipis out-of-bounds access
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <1535521943-5547-1-git-send-email-wanpengli@tencent.com>
Date:   Wed, 29 Aug 2018 12:05:06 +0300
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        =?utf-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Dan Carpenter <dan.carpenter@oracle.com>
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Message-Id: <A523C29C-C3E8-4FEF-8620-96365F774582@oracle.com>
References: <1535521943-5547-1-git-send-email-wanpengli@tencent.com>
To:     Wanpeng Li <kernellwp@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On 29 Aug 2018, at 8:52, Wanpeng Li <kernellwp@gmail.com> wrote:
>=20
> From: Wanpeng Li <wanpengli@tencent.com>
>=20
> Dan Carpenter reported that the untrusted data returns from =
kvm_register_read()
> results in the following static checker warning:
>  arch/x86/kvm/lapic.c:576 kvm_pv_send_ipi()
>  error: buffer underflow 'map->phys_map' 's32min-s32max'
>=20
> KVM guest can easily trigger this by executing the following assembly =
sequence=20
> in Ring0:
>=20
> mov $10, %rax
> mov $0xFFFFFFFF, %rbx
> mov $0xFFFFFFFF, %rdx
> mov $0, %rsi
> vmcall
>=20
> As this will cause KVM to execute the following code-path:
> vmx_handle_exit() -> handle_vmcall() -> kvm_emulate_hypercall() -> =
kvm_pv_send_ipi()
> which will reach out-of-bounds access.
>=20
> This patch fixes it by adding a check to kvm_pv_send_ipi() against =
map->max_apic_id=20
> and also checking whether or not map->phys_map[min + i] is NULL since =
the max_apic_id=20
> is set according to the max apic id, however, some phys_map maybe NULL =
when apic id=20
> is sparse, in addition, kvm also unconditionally set max_apic_id to =
255 to reserve=20
> enough space for any xAPIC ID.
>=20
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Kr=C4=8Dm=C3=A1=C5=99 <rkrcmar@redhat.com>
> Cc: Liran Alon <liran.alon@oracle.com>
> Cc: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> arch/x86/kvm/lapic.c | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 deletions(-)
>=20
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 0cefba2..86e933c 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -571,18 +571,27 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned =
long ipi_bitmap_low,
> 	rcu_read_lock();
> 	map =3D rcu_dereference(kvm->arch.apic_map);
>=20
> +	if (unlikely((s32)(map->max_apic_id - __fls(ipi_bitmap_low)) < =
min))
> +		goto out;

I personally think =E2=80=9Cif ((min + __fls(ipi_bitmap_low)) > =
map->max_apic_id)=E2=80=9D is more readable.
But that=E2=80=99s just a matter of taste :)

> 	/* Bits above cluster_size are masked in the caller.  */
> 	for_each_set_bit(i, &ipi_bitmap_low, BITS_PER_LONG) {
> -		vcpu =3D map->phys_map[min + i]->vcpu;
> -		count +=3D kvm_apic_set_irq(vcpu, &irq, NULL);
> +		if (map->phys_map[min + i]) {
> +			vcpu =3D map->phys_map[min + i]->vcpu;
> +			count +=3D kvm_apic_set_irq(vcpu, &irq, NULL);
> +		}
> 	}
>=20
> 	min +=3D cluster_size;
> +	if (unlikely((s32)(map->max_apic_id - __fls(ipi_bitmap_high)) < =
min))
> +		goto out;
> 	for_each_set_bit(i, &ipi_bitmap_high, BITS_PER_LONG) {
> -		vcpu =3D map->phys_map[min + i]->vcpu;
> -		count +=3D kvm_apic_set_irq(vcpu, &irq, NULL);
> +		if (map->phys_map[min + i]) {
> +			vcpu =3D map->phys_map[min + i]->vcpu;
> +			count +=3D kvm_apic_set_irq(vcpu, &irq, NULL);
> +		}
> 	}
>=20
> +out:
> 	rcu_read_unlock();
> 	return count;
> }
> --=20
> 2.7.4
>=20

Reviewed-By: Liran Alon <liran.alon@oracle.com>