From: Sean Christopherson <[email protected]>
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 <[email protected]>
Cc: Rick Edgecombe <[email protected]>
Cc: Peter Zijlstra (Intel) <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: [email protected]
Fixes: f2fde6a5bcfcf ("KVM: VMX: Move RSB stuffing to before the first RET
after VM-Exit")
Signed-off-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
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 @@ 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
