This is a note to let you know that I've just added the patch titled
KVM: VMX: Explicitly clear RFLAGS.CF and RFLAGS.ZF in VM-Exit RSB path
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
kvm-vmx-explicitly-clear-rflags.cf-and-rflags.zf-in-vm-exit-rsb-path.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From c7cb2d650c9e78c03bd2d1c0db89891825f8c0f4 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Date: Tue, 5 May 2020 20:53:55 -0700
Subject: KVM: VMX: Explicitly clear RFLAGS.CF and RFLAGS.ZF in VM-Exit RSB path
From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
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@xxxxxx>
Cc: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: f2fde6a5bcfcf ("KVM: VMX: Move RSB stuffing to before the first RET after VM-Exit")
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Message-Id: <20200506035355.2242-1-sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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
Patches currently in stable-queue which might be from sean.j.christopherson@xxxxxxxxx are
queue-5.4/kvm-vmx-explicitly-clear-rflags.cf-and-rflags.zf-in-vm-exit-rsb-path.patch
