linux-kernel - [PATCH v12 105/106] RFC: KVM: TDX: Make busy with S-EPT on entry bug

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Date:   Mon, 27 Feb 2023 00:23:44 -0800
From:   isaku.yamahata@...el.com
To:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...el.com, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
        Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Kai Huang <kai.huang@...el.com>,
        Zhi Wang <zhi.wang.linux@...il.com>
Subject: [PATCH v12 105/106] RFC: KVM: TDX: Make busy with S-EPT on entry bug

From: Isaku Yamahata <isaku.yamahata@...el.com>

TDX module has mitigation against zero-step attacks or single-step attacks.
When the TDX module finds repeated EPT violations on the same guest RIP,
i.e. no advance in guest, it starts to suspect the attack.  The mitigation
logic on the next entry tries to take the lock of S-EPT.  It may result in
an error of TDX_OPERAND_BUSY | TDX_OPERAND_ID_SEPT.  As KVM shouldn't
spuriously zap private S-EPT so that guest can make progress, KVM
shouldn't cause the TDX module to trigger the mitigation.  Make
(TDX_OPERAND_BUSY | TDX_OPERAND_ID_SEPT) on entry KVM bug.

Suggested-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
---
- This patch is RFC because this is only lightly tested and stress test
  isn't done.
---
 arch/x86/kvm/vmx/tdx.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 292f55efe8f7..846cd4255f49 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1712,8 +1712,20 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
 {
 	union tdx_exit_reason exit_reason = to_tdx(vcpu)->exit_reason;
 
-	/* See the comment of tdh_sept_seamcall(). */
-	if (unlikely(exit_reason.full == (TDX_OPERAND_BUSY | TDX_OPERAND_ID_SEPT)))
+	/*
+	 * See the comment of tdh_sept_seamcall().
+	 * TDX module has mitigation against zero-step attacks or single-step
+	 * attacks.  When the TDX module finds repeated EPT violations on the
+	 * same guest RIP, i.e. no advance in guest, it starts to suspect the
+	 * attack.  The mitigation logic on the next entry tries to take the
+	 * lock of S-EPT.  It may result in an error of (TDX_OPERAND_BUSY |
+	 * TDX_OPERAND_ID_SEPT).  As KVM shouldn't spuriously zap private S-EPT
+	 * so that guest can make progress, KVM shouldn't cause the TDX module
+	 * to trigger the mitigation.  Make (TDX_OPERAND_BUSY |
+	 * TDX_OPERAND_ID_SEPT) on entry KVM bug.
+	 */
+	if (KVM_BUG_ON(exit_reason.full == (TDX_OPERAND_BUSY | TDX_OPERAND_ID_SEPT),
+		       vcpu->kvm))
 		return 1;
 
 	if (unlikely(exit_reason.non_recoverable || exit_reason.error)) {
-- 
2.25.1