[02/10] KVM: SVM: Don't rely on DebugSwap to restore host DR0..DR3

Commit Message

Sean Christopherson Feb. 19, 2025, 1:26 a.m. UTC

Never rely on the CPU to restore/load host DR0..DR3 values, even if the
CPU supports DebugSwap, as there are no guarantees that SNP guests will
actually enable DebugSwap on APs.  E.g. if KVM were to rely on the CPU to
load DR0..DR3 and skipped them during hw_breakpoint_restore(), KVM would
run with clobbered-to-zero DRs if an SNP guest created APs without
DebugSwap enabled.

Update the comment to explain the dangers, and hopefully prevent breaking
KVM in the future.

Signed-off-by: Sean Christopherson <[e-mail]>
---
 arch/x86/kvm/svm/sev.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

Patch

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index e3606d072735..6c6d45e13858 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4594,18 +4594,21 @@  void sev_es_prepare_switch_to_guest(struct vcpu_svm *svm, struct sev_es_save_are
 	/*
 	 * If DebugSwap is enabled, debug registers are loaded but NOT saved by
 	 * the CPU (Type-B). If DebugSwap is disabled/unsupported, the CPU both
-	 * saves and loads debug registers (Type-A).  Sadly, on CPUs without
-	 * ALLOWED_SEV_FEATURES, KVM can't prevent SNP guests from enabling
-	 * DebugSwap on secondary vCPUs without KVM's knowledge via "AP Create",
-	 * and so KVM must save DRs if DebugSwap is supported to prevent DRs
-	 * from being clobbered by a misbehaving guest.
+	 * saves and loads debug registers (Type-A).  Sadly, KVM can't prevent
+	 * SNP guests from lying about DebugSwap on secondary vCPUs, i.e. the
+	 * SEV_FEATURES provided at "AP Create" isn't guaranteed to match what
+	 * the guest has actually enabled (or not!) in the VMSA.
+	 *
+	 * If DebugSwap is *possible*, save the masks so that they're restored
+	 * if the guest enables DebugSwap.  But for the DRs themselves, do NOT
+	 * rely on the CPU to restore the host values; KVM will restore them as
+	 * needed in common code, via hw_breakpoint_restore().  Note, KVM does
+	 * NOT support virtualizing Breakpoint Extensions, i.e. the mask MSRs
+	 * don't need to be restored per se, KVM just needs to ensure they are
+	 * loaded with the correct values *if* the CPU writes the MSRs.
 	 */
 	if (sev_vcpu_has_debug_swap(svm) ||
 	    (sev_snp_guest(kvm) && cpu_feature_enabled(X86_FEATURE_DEBUG_SWAP))) {
-		hostsa->dr0 = native_get_debugreg(0);
-		hostsa->dr1 = native_get_debugreg(1);
-		hostsa->dr2 = native_get_debugreg(2);
-		hostsa->dr3 = native_get_debugreg(3);
 		hostsa->dr0_addr_mask = amd_get_dr_addr_mask(0);
 		hostsa->dr1_addr_mask = amd_get_dr_addr_mask(1);
 		hostsa->dr2_addr_mask = amd_get_dr_addr_mask(2);