From: Sean Christopherson sean.j.christopherson@intel.com
commit c7cb2d650c9e78c03bd2d1c0db89891825f8c0f4 upstream.
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 cai@lca.pw Cc: Rick Edgecombe rick.p.edgecombe@intel.com Cc: Peter Zijlstra (Intel) peterz@infradead.org Cc: Josh Poimboeuf jpoimboe@redhat.com 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 sean.j.christopherson@intel.com Message-Id: 20200506035355.2242-1-sean.j.christopherson@intel.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/kvm/vmx/vmenter.S | 3 +++ 1 file changed, 3 insertions(+)
--- a/arch/x86/kvm/vmx/vmenter.S +++ b/arch/x86/kvm/vmx/vmenter.S @@ -86,6 +86,9 @@ ENTRY(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