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([2001:b07:6468:f312:11d7:2f21:f38b:17e]) by smtp.gmail.com with ESMTPSA id d27sm2027867wra.30.2020.05.06.03.03.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 06 May 2020 03:03:46 -0700 (PDT) Subject: Re: [PATCH] KVM: VMX: Explicitly clear RFLAGS.CF and RFLAGS.ZF in VM-Exit RSB path To: Sean Christopherson Cc: Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Qian Cai , Rick Edgecombe , Peter Zijlstra , Josh Poimboeuf References: <20200506035355.2242-1-sean.j.christopherson@intel.com> From: Paolo Bonzini Message-ID: <1f906b20-4806-4de0-fa99-9e7607464eb0@redhat.com> Date: Wed, 6 May 2020 12:03:45 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200506035355.2242-1-sean.j.christopherson@intel.com> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > Cc: Rick Edgecombe > Cc: Peter Zijlstra (Intel) > Cc: Josh Poimboeuf > Cc: stable@vger.kernel.org > Fixes: f2fde6a5bcfcf ("KVM: VMX: Move RSB stuffing to before the first RET after VM-Exit") > Signed-off-by: Sean Christopherson > --- > 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