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FAILED: patch "[PATCH] KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted" failed to apply to 6.6-stable tree

PATCH [email protected] 1 participant 1 message May 3

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Stable branch x86 KVM x86 KVM x86 Hyper-V
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The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x c36991c6f8d2ab56ee67aff04e3c357f45cfc76c
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026050356-carnation-knelt-8b88@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From c36991c6f8d2ab56ee67aff04e3c357f45cfc76c Mon Sep 17 00:00:00 2001
From: Kevin Cheng <[email protected]>
Date: Tue, 3 Mar 2026 16:22:22 -0800
Subject: [PATCH] KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted
by L1

Explicitly synthesize a #UD for VMMCALL if L2 is active, L1 does NOT want to intercept VMMCALL, nested_svm_l2_tlb_flush_enabled() is true, and the hypercall is something other than one of the supported Hyper-V hypercalls. When all of the above conditions are met, KVM will intercept VMMCALL but never forward it to L1, i.e. will let L2 make hypercalls as if it were L1.

The TLFS says a whole lot of nothing about this scenario, so go with the architectural behavior, which says that VMMCALL #UDs if it's not intercepted.

Opportunistically do a 2-for-1 stub trade by stub-ifying the new API instead of the helpers it uses. The last remaining "single" stub will soon be dropped as well.

Suggested-by Sean Christopherson <[email protected]>
Fixes 3f4a812edf5c ("KVM: nSVM: hyper-v: Enable L2 TLB flush")
Cc Vitaly Kuznetsov <[email protected]>
Cc [email protected]
Signed-off-by Kevin Cheng <[email protected]>
Link https://patch.msgid.link/[email protected] open here
[sean: rewrite changelog and comment, tag for stable, remove defunct stubs]
Reviewed-by Yosry Ahmed <[email protected]>
Reviewed-by Vitaly Kuznetsov <[email protected]>
Link https://patch.msgid.link/[email protected] open here
Signed-off-by Sean Christopherson <[email protected]>
arch/x86/kvm/hyperv.h +0 -8
index 6ce160ffa678..6301f79fcbae 100644
@@ -305,14 +305,6 @@ static inline bool kvm_hv_has_stimer_pending(struct kvm_vcpu *vcpu)
 {
 	return false;
 }
-static inline bool kvm_hv_is_tlb_flush_hcall(struct kvm_vcpu *vcpu)
-{
-	return false;
-}
-static inline bool guest_hv_cpuid_has_l2_tlb_flush(struct kvm_vcpu *vcpu)
-{
-	return false;
-}
 static inline int kvm_hv_verify_vp_assist(struct kvm_vcpu *vcpu)
 {
 	return 0;
arch/x86/kvm/svm/hyperv.h +11 -0
index d3f8bfc05832..9af03970d40c 100644
@@ -41,6 +41,13 @@ static inline bool nested_svm_l2_tlb_flush_enabled(struct kvm_vcpu *vcpu)
 	return hv_vcpu->vp_assist_page.nested_control.features.directhypercall;
 }
 
+static inline bool nested_svm_is_l2_tlb_flush_hcall(struct kvm_vcpu *vcpu)
+{
+	return guest_hv_cpuid_has_l2_tlb_flush(vcpu) &&
+	       nested_svm_l2_tlb_flush_enabled(vcpu) &&
+	       kvm_hv_is_tlb_flush_hcall(vcpu);
+}
+
 void svm_hv_inject_synthetic_vmexit_post_tlb_flush(struct kvm_vcpu *vcpu);
 #else /* CONFIG_KVM_HYPERV */
 static inline void nested_svm_hv_update_vm_vp_ids(struct kvm_vcpu *vcpu) {}

@@ -48,6 +55,10 @@ static inline bool nested_svm_l2_tlb_flush_enabled(struct kvm_vcpu *vcpu)
 {
 	return false;
 }
+static inline bool nested_svm_is_l2_tlb_flush_hcall(struct kvm_vcpu *vcpu)
+{
+	return false;
+}
 static inline void svm_hv_inject_synthetic_vmexit_post_tlb_flush(struct kvm_vcpu *vcpu) {}
 #endif /* CONFIG_KVM_HYPERV */
arch/x86/kvm/svm/nested.c +1 -3
index 5b639d98bf09..0f7893a7cb04 100644
@@ -1738,9 +1738,7 @@ int nested_svm_exit_special(struct vcpu_svm *svm)
 	}
 	case SVM_EXIT_VMMCALL:
 		/* Hyper-V L2 TLB flush hypercall is handled by L0 */
-		if (guest_hv_cpuid_has_l2_tlb_flush(vcpu) &&
-		    nested_svm_l2_tlb_flush_enabled(vcpu) &&
-		    kvm_hv_is_tlb_flush_hcall(vcpu))
+		if (nested_svm_is_l2_tlb_flush_hcall(vcpu))
 			return NESTED_EXIT_HOST;
 		break;
 	default:
arch/x86/kvm/svm/svm.c +18 -1
index 7efa71709292..9e6864cf58d3 100644
@@ -52,6 +52,7 @@
 #include "svm.h"
 #include "svm_ops.h"
 
+#include "hyperv.h"
 #include "kvm_onhyperv.h"
 #include "svm_onhyperv.h"
 

@@ -3248,6 +3249,22 @@ static int bus_lock_exit(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
+static int vmmcall_interception(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * Inject a #UD if L2 is active and the VMMCALL isn't a Hyper-V TLB
+	 * hypercall, as VMMCALL #UDs if it's not intercepted, and this path is
+	 * reachable if and only if L1 doesn't want to intercept VMMCALL or has
+	 * enabled L0 (KVM) handling of Hyper-V L2 TLB flush hypercalls.
+	 */
+	if (is_guest_mode(vcpu) && !nested_svm_is_l2_tlb_flush_hcall(vcpu)) {
+		kvm_queue_exception(vcpu, UD_VECTOR);
+		return 1;
+	}
+
+	return kvm_emulate_hypercall(vcpu);
+}
+
 static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
 	[SVM_EXIT_READ_CR0]			= cr_interception,
 	[SVM_EXIT_READ_CR3]			= cr_interception,

@@ -3298,7 +3315,7 @@ static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
 	[SVM_EXIT_TASK_SWITCH]			= task_switch_interception,
 	[SVM_EXIT_SHUTDOWN]			= shutdown_interception,
 	[SVM_EXIT_VMRUN]			= vmrun_interception,
-	[SVM_EXIT_VMMCALL]			= kvm_emulate_hypercall,
+	[SVM_EXIT_VMMCALL]			= vmmcall_interception,
 	[SVM_EXIT_VMLOAD]			= vmload_interception,
 	[SVM_EXIT_VMSAVE]			= vmsave_interception,
 	[SVM_EXIT_STGI]				= stgi_interception,