linux-kernel - [PATCH 4.19 087/190] KVM: VMX: Compare only a single byte for VMCS "launched" in vCPU-run

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Date:   Fri, 13 Sep 2019 14:05:42 +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>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.19 087/190] KVM: VMX: Compare only a single byte for VMCS "launched" in vCPU-run

[ Upstream commit 61c08aa9606d4e48a8a50639c956448a720174c3 ]

The vCPU-run asm blob does a manual comparison of a VMCS' launched
status 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].

Originally, 'launched' was of type 'int' and so the asm blob used 'cmpl'
to check the launch status.  When 'launched' was moved to be stored on a
per-VMCS basis, struct vcpu_vmx's "temporary" __launched flag was added
in order to avoid having to pass the current VMCS into the asm blob.
The new  '__launched' was defined as a 'bool' and not an 'int', but the
'cmp' instruction was not updated.

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: d462b8192368 ("KVM: VMX: Keep list of loaded VMCSs, instead of vcpus")
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: Sasha Levin <sashal@...nel.org>
---
 arch/x86/kvm/vmx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 2e310ea62d609..562f5dc4645b6 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -10808,7 +10808,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 		"mov %%" _ASM_AX", %%cr2 \n\t"
 		"3: \n\t"
 		/* Check if vmlaunch of vmresume is needed */
-		"cmpl $0, %c[launched](%0) \n\t"
+		"cmpb $0, %c[launched](%0) \n\t"
 		/* Load guest registers.  Don't clobber flags. */
 		"mov %c[rax](%0), %%" _ASM_AX " \n\t"
 		"mov %c[rbx](%0), %%" _ASM_BX " \n\t"
-- 
2.20.1