linux-kernel - [PATCH] SEV: disable SEV-ES DebugSwap by default

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Date: Sat,  9 Mar 2024 11:31:14 -0500
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc: seanjc@...gle.com,
	stable@...r.kernel.org
Subject: [PATCH] SEV: disable SEV-ES DebugSwap by default

The DebugSwap feature of SEV-ES provides a way for confidential guests to use
data breakpoints.  However, because the status of the DebugSwap feature is
recorded in the VMSA, enabling it by default invalidates the attestation
signatures.  In 6.10 we will introduce a new API to create SEV VMs that
will allow enabling DebugSwap based on what the user tells KVM to do.
Contextually, we will change the legacy KVM_SEV_ES_INIT API to never
enable DebugSwap.

For compatibility with kernels that pre-date the introduction of DebugSwap,
as well as with those where KVM_SEV_ES_INIT will never enable it, do not enable
the feature by default.  If anybody wants to use it, for now they can enable
the sev_es_debug_swap_enabled module parameter, but this will result in a
warning.

Fixes: d1f85fbe836e ("KVM: SEV: Enable data breakpoints in SEV-ES")
Cc: stable@...r.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm/sev.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index f760106c31f8..69b37956c1c8 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -57,7 +57,7 @@ static bool sev_es_enabled = true;
 module_param_named(sev_es, sev_es_enabled, bool, 0444);

 /* enable/disable SEV-ES DebugSwap support */
-static bool sev_es_debug_swap_enabled = true;
+static bool sev_es_debug_swap_enabled = false;
 module_param_named(debug_swap, sev_es_debug_swap_enabled, bool, 0444);
 #else
 #define sev_enabled false
@@ -612,8 +612,11 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
 	save->xss  = svm->vcpu.arch.ia32_xss;
 	save->dr6  = svm->vcpu.arch.dr6;

-	if (sev_es_debug_swap_enabled)
+	if (sev_es_debug_swap_enabled) {
 		save->sev_features |= SVM_SEV_FEAT_DEBUG_SWAP;
+		pr_warn_once("Enabling DebugSwap with KVM_SEV_ES_INIT. "
+			     "This will not work starting with Linux 6.10\n");
+	}

 	pr_debug("Virtual Machine Save Area (VMSA):\n");
 	print_hex_dump_debug("", DUMP_PREFIX_NONE, 16, 1, save, sizeof(*save), false);
-- 
2.39.1