Re: [PATCH] KVM: nVMX: Ignore segment base for VMX memory operand when segment not FS or GS

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Liran Alon <liran.alon@oracle.com>
Cc: max@m00nbsd.net, Joao Martins <joao.m.martins@oracle.com>,
	pbonzini@redhat.com, rkrcmar@redhat.com, kvm@vger.kernel.org
Subject: Re: [PATCH] KVM: nVMX: Ignore segment base for VMX memory operand when segment not FS or GS
Date: Mon, 15 Jul 2019 18:22:52 +0200	[thread overview]
Message-ID: <87r26rw9lv.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20190715154744.36134-1-liran.alon@oracle.com>

Liran Alon <liran.alon@oracle.com> writes:

> As reported by Maxime at
> https://bugzilla.kernel.org/show_bug.cgi?id=204175:
>
> In vmx/nested.c::get_vmx_mem_address(), when the guest runs in long mode,
> the base address of the memory operand is computed with a simple:
>     *ret = s.base + off;
>
> This is incorrect, the base applies only to FS and GS, not to the others.
> Because of that, if the guest uses a VMX instruction based on DS and has
> a DS.base that is non-zero, KVM wrongfully adds the base to the
> resulting address.
>
> Reported-by: Maxime Villard <max@m00nbsd.net>
> Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 18efb338ed8a..e01e1b6b8167 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4068,6 +4068,8 @@ int get_vmx_mem_address(struct kvm_vcpu *vcpu, unsigned long exit_qualification,
>  		 * mode, e.g. a 32-bit address size can yield a 64-bit virtual
>  		 * address when using FS/GS with a non-zero base.
>  		 */
> +		if ((seg_reg != VCPU_SREG_FS) && (seg_reg != VCPU_SREG_GS))
> +			s.base = 0;

(personal preference)

 I'd rather write this as

    /* In long mode only FS and GS bases are considered */
    if (seg_reg == VCPU_SREG_FS || seg_reg == VCPU_SREG_GS)
       *ret = s.base + off;
    else 
       *ret = off;

>  		*ret = s.base + off;
>  
>  		/* Long mode: #GP(0)/#SS(0) if the memory address is in a

As-is or rewritten with my suggestion,

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly