diff mbox series

[v2,6/6] KVM: VMX: Move VERW closer to VMentry for MDS mitigation

Message ID 20231024-delay-verw-v2-6-f1881340c807@linux.intel.com
State New
Headers
Series Delay VERW |

Commit Message

Pawan Gupta Oct. 24, 2023, 8:08 a.m. UTC 
  During VMentry VERW is executed to mitigate MDS. After VERW, any memory
access like register push onto stack may put host data in MDS affected
CPU buffers. A guest can then use MDS to sample host data.

Although likelihood of secrets surviving in registers at current VERW
callsite is less, but it can't be ruled out. Harden the MDS mitigation
by moving the VERW mitigation late in VMentry path.

Note that VERW for MMIO Stale Data mitigation is unchanged because of
the complexity of per-guest conditional VERW which is not easy to handle
that late in asm with no GPRs available. If the CPU is also affected by
MDS, VERW is unconditionally executed late in asm regardless of guest
having MMIO access.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/kvm/vmx/vmenter.S |  4 ++++
 arch/x86/kvm/vmx/vmx.c     | 10 +++++++---
 2 files changed, 11 insertions(+), 3 deletions(-)

Comments

Chao Gao Oct. 25, 2023, 7:47 a.m. UTC | #1 
On Tue, Oct 24, 2023 at 01:08:53AM -0700, Pawan Gupta wrote:
>During VMentry VERW is executed to mitigate MDS. After VERW, any memory
>access like register push onto stack may put host data in MDS affected
>CPU buffers. A guest can then use MDS to sample host data.
>
>Although likelihood of secrets surviving in registers at current VERW
>callsite is less, but it can't be ruled out. Harden the MDS mitigation
>by moving the VERW mitigation late in VMentry path.
>
>Note that VERW for MMIO Stale Data mitigation is unchanged because of
>the complexity of per-guest conditional VERW which is not easy to handle
>that late in asm with no GPRs available. If the CPU is also affected by
>MDS, VERW is unconditionally executed late in asm regardless of guest
>having MMIO access.
>
>Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>---
> arch/x86/kvm/vmx/vmenter.S |  4 ++++
> arch/x86/kvm/vmx/vmx.c     | 10 +++++++---
> 2 files changed, 11 insertions(+), 3 deletions(-)
>
>diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
>index b3b13ec04bac..c566035938cc 100644
>--- a/arch/x86/kvm/vmx/vmenter.S
>+++ b/arch/x86/kvm/vmx/vmenter.S
>@@ -1,6 +1,7 @@
> /* SPDX-License-Identifier: GPL-2.0 */
> #include <linux/linkage.h>
> #include <asm/asm.h>
>+#include <asm/segment.h>

This header is already included a few lines below:

#include <asm/nospec-branch.h>
#include <asm/percpu.h>
#include <asm/segment.h>	<---

> #include <asm/bitsperlong.h>
> #include <asm/kvm_vcpu_regs.h>
> #include <asm/nospec-branch.h>
Pawan Gupta Oct. 25, 2023, 3:15 p.m. UTC | #2 
On Wed, Oct 25, 2023 at 03:47:37PM +0800, Chao Gao wrote:
> >diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> >index b3b13ec04bac..c566035938cc 100644
> >--- a/arch/x86/kvm/vmx/vmenter.S
> >+++ b/arch/x86/kvm/vmx/vmenter.S
> >@@ -1,6 +1,7 @@
> > /* SPDX-License-Identifier: GPL-2.0 */
> > #include <linux/linkage.h>
> > #include <asm/asm.h>
> >+#include <asm/segment.h>
> 
> This header is already included a few lines below:

Thanks, will remove it.
diff mbox series

Patch

diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index b3b13ec04bac..c566035938cc 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -1,6 +1,7 @@ 
 /* SPDX-License-Identifier: GPL-2.0 */
 #include <linux/linkage.h>
 #include <asm/asm.h>
+#include <asm/segment.h>
 #include <asm/bitsperlong.h>
 #include <asm/kvm_vcpu_regs.h>
 #include <asm/nospec-branch.h>
@@ -161,6 +162,9 @@  SYM_FUNC_START(__vmx_vcpu_run)
 	/* Load guest RAX.  This kills the @regs pointer! */
 	mov VCPU_RAX(%_ASM_AX), %_ASM_AX
 
+	/* Clobbers EFLAGS.ZF */
+	CLEAR_CPU_BUFFERS
+
 	/* Check EFLAGS.CF from the VMX_RUN_VMRESUME bit test above. */
 	jnc .Lvmlaunch
 
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 24e8694b83fc..e2234c0643e9 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7226,13 +7226,17 @@  static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 
 	guest_state_enter_irqoff();
 
-	/* L1D Flush includes CPU buffer clear to mitigate MDS */
+	/*
+	 * L1D Flush includes CPU buffer clear to mitigate MDS, but VERW
+	 * mitigation for MDS is done late in VMentry and is still
+	 * executed inspite of L1D Flush. This is because an extra VERW
+	 * should not matter much after the big hammer L1D Flush.
+	 */
 	if (static_branch_unlikely(&vmx_l1d_should_flush))
 		vmx_l1d_flush(vcpu);
-	else if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF))
-		mds_clear_cpu_buffers();
 	else if (static_branch_unlikely(&mmio_stale_data_clear) &&
 		 kvm_arch_has_assigned_device(vcpu->kvm))
+		/* MMIO mitigation is mutually exclusive to MDS mitigation later in asm */
 		mds_clear_cpu_buffers();
 
 	vmx_disable_fb_clear(vmx);