linux-kernel - [PATCH 5.0 232/238] KVM: nVMX: Check a single byte for VMCS "launched" in nested early checks

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Message-Id: <20190322111312.070930068@linuxfoundation.org>
Date:   Fri, 22 Mar 2019 12:17:31 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 5.0 232/238] KVM: nVMX: Check a single byte for VMCS "launched" in nested early checks

5.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Sean Christopherson <sean.j.christopherson@...el.com>

commit 1ce072cbfd8dba46f117804850398e0b3040a541 upstream.

Nested early checks does a manual comparison of a VMCS' launched status
in its asm blob to execute the correct VM-Enter instruction, i.e.
VMLAUNCH vs. VMRESUME.  The launched flag is a bool, which is a typedef
of _Bool.  C99 does not define an exact size for _Bool, stating only
that is must be large enough to hold '0' and '1'.  Most, if not all,
compilers use a single byte for _Bool, including gcc[1].

The use of 'cmpl' instead of 'cmpb' was not deliberate, but rather the
result of a copy-paste as the asm blob was directly derived from the asm
blob for vCPU-run.

This has not caused any known problems, likely due to compilers aligning
variables to 4-byte or 8-byte boundaries and KVM zeroing out struct
vcpu_vmx during allocation.  I.e. vCPU-run accesses "junk" data, it just
happens to always be zero and so doesn't affect the result.

[1] https://gcc.gnu.org/ml/gcc-patches/2000-10/msg01127.html

Fixes: 52017608da33 ("KVM: nVMX: add option to perform early consistency checks via H/W")
Cc: <stable@...r.kernel.org>
Reviewed-by: Jim Mattson <jmattson@...gle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/kvm/vmx/nested.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2765,7 +2765,7 @@ static int nested_vmx_check_vmentry_hw(s
 		"add $%c[wordsize], %%" _ASM_SP "\n\t" /* un-adjust RSP */

 		/* Check if vmlaunch or vmresume is needed */
-		"cmpl $0, %c[launched](%% " _ASM_CX")\n\t"
+		"cmpb $0, %c[launched](%% " _ASM_CX")\n\t"

 		"call vmx_vmenter\n\t"