linux-kernel - Re: [PATCH] KVM: VMX: Explicitly clear RFLAGS.CF and RFLAGS.ZF in VM-Exit RSB path

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Date:   Wed, 6 May 2020 12:03:45 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Qian Cai <cai@....pw>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH] KVM: VMX: Explicitly clear RFLAGS.CF and RFLAGS.ZF in
 VM-Exit RSB path

On 06/05/20 05:53, Sean Christopherson wrote:
> Clear CF and ZF in the VM-Exit path after doing __FILL_RETURN_BUFFER so
> that KVM doesn't interpret clobbered RFLAGS as a VM-Fail.  Filling the
> RSB has always clobbered RFLAGS, its current incarnation just happens
> clear CF and ZF in the processs.  Relying on the macro to clear CF and
> ZF is extremely fragile, e.g. commit 089dd8e53126e ("x86/speculation:
> Change FILL_RETURN_BUFFER to work with objtool") tweaks the loop such
> that the ZF flag is always set.
> 
> Reported-by: Qian Cai <cai@....pw>
> Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: stable@...r.kernel.org
> Fixes: f2fde6a5bcfcf ("KVM: VMX: Move RSB stuffing to before the first RET after VM-Exit")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/vmx/vmenter.S | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index 87f3f24fef37b..51d1a82742fd5 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S
> @@ -82,6 +82,9 @@ SYM_FUNC_START(vmx_vmexit)
>  	/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
>  	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
>  
> +	/* Clear RFLAGS.CF and RFLAGS.ZF to preserve VM-Exit, i.e. !VM-Fail. */
> +	or $1, %_ASM_AX
> +
>  	pop %_ASM_AX
>  .Lvmexit_skip_rsb:
>  #endif
> 

Queued, thanks (for 5.7 so that it will never be broken in Linus's tree).

Paolo