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

Greg Kroah-Hartman
2019-09-13T13:31:20Z

[ 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@vger.kernel.org>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.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