From c7cb2d650c9e78c03bd2d1c0db89891825f8c0f4 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Tue, 5 May 2020 20:53:55 -0700 Subject: [PATCH] KVM: VMX: Explicitly clear RFLAGS.CF and RFLAGS.ZF in VM-Exit RSB path 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 Message-Id: <20200506035355.2242-1-sean.j.christopherson@intel.com> Signed-off-by: Paolo Bonzini --- 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 87f3f24fef37..51d1a82742fd 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