[PATCH] KVM: VMX: Explicitly clear RFLAGS.CF and RFLAGS.ZF in VM-Exit RSB path

[PATCH] KVM: VMX: Explicitly clear RFLAGS.CF and RFLAGS.ZF in VM-Exit RSB path

[Sean Christopherson]

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>
---
 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 87f3f24fef37b..51d1a82742fd5 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
-- 
2.26.0

Re: [PATCH] KVM: VMX: Explicitly clear RFLAGS.CF and RFLAGS.ZF in VM-Exit RSB path

[Paolo Bonzini]

On 06/05/20 05:53, Sean Christopherson wrote:
> 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>
> ---
>  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 87f3f24fef37b..51d1a82742fd5 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
> 

Queued, thanks (for 5.7 so that it will never be broken in Linus's tree).

Paolo