linux-kernel - Re: [PATCH 03/22] x86/kvm: Fix frame pointer usage in vmx

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Message-ID: <20190715133525.gr4wvnd4kxwtv63o@treble>
Date:   Mon, 15 Jul 2019 08:35:25 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 03/22] x86/kvm: Fix frame pointer usage in vmx_vmenter()

On Mon, Jul 15, 2019 at 03:03:23PM +0200, Paolo Bonzini wrote:
> On 15/07/19 14:37, Josh Poimboeuf wrote:
> > On Mon, Jul 15, 2019 at 11:04:03AM +0200, Paolo Bonzini wrote:
> >> On 15/07/19 02:36, Josh Poimboeuf wrote:
> >>> With CONFIG_FRAME_POINTER, vmx_vmenter() needs to do frame pointer setup
> >>> before calling kvm_spurious_fault().
> >>>
> >>> Fixes the following warning:
> >>>
> >>>   arch/x86/kvm/vmx/vmenter.o: warning: objtool: vmx_vmenter()+0x14: call without frame pointer save/setup
> >>>
> >>> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> >>
> >> This is not enough, because the RSP value must match what is computed at
> >> this place:
> >>
> >>         /* Adjust RSP to account for the CALL to vmx_vmenter(). */
> >>         lea -WORD_SIZE(%_ASM_SP), %_ASM_ARG2
> >>         call vmx_update_host_rsp
> > 
> > Ah, that is surprising :-)
> > 
> > And then there's this, which overwrites the frame pointer anyway:
> > 
> > 	mov VCPU_RBP(%_ASM_AX), %_ASM_BP
> > 
> > Would it make sense to remove the call to vmx_vmenter() altogether, and
> > just either embed it in __vmx_vcpu_run(), or jmp back and forth to it
> > from __vmx_vcpu_run()?
> 
> Unfortunately there's another use of it in nested_vmx_check_vmentry_hw.

Ah, I missed that too (failed by cscope).  Is this ok?

diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index d4cb1945b2e3..4010d519eb8c 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -54,9 +54,9 @@ ENTRY(vmx_vmenter)
 	ret
 
 3:	cmpb $0, kvm_rebooting
-	jne 4f
-	call kvm_spurious_fault
-4:	ret
+	je 4f
+	ret
+4:	ud2
 
 	.pushsection .fixup, "ax"
 5:	jmp 3b