FAILED: patch "[PATCH] KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put()" failed to apply to 6.18-stable tree

PATCH New [email protected] 2 participants 3 messages Sep 9 – Sep 11

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author maintainer

2026-09-09 10:45 UTC #0 lore raw

The patch below does not apply to the 6.18-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-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 38640bc32be3fcf9526d477155bc19d3f146231f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090924-dweeb-huskiness-bd94@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

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

From 38640bc32be3fcf9526d477155bc19d3f146231f Mon Sep 17 00:00:00 2001
From: Marc Zyngier <[email protected]>
Date: Thu, 6 Aug 2026 10:10:23 +0100
Subject: [PATCH] KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put()
VNCR unmapping

While VNCR TLB invalidation always occurs under the MMU lock, vcpu_put() doesn't, while it unmaps the VNCR page.

The problem is that the invalidation evaluates vncr_tlb::cpu to decide whether an unmapping needs to take place (cpu != -1) before performing it. On the other hand, this_cpu_reset_vncr_fixmap() unconditionally unmaps if L1_VNCR_MAPPED is set.

These two obviously can race, with a TOCTOU pattern on the TLBI path, and a BUG_ON() on the vcpu_put() path. And the two can end-up calling vncr_fixmap(-1), with extra lethal effects.

Move the reset of vncr_tlb::cpu to -1 to a common function, and make this update atomic so that only a single thread can reset the field and perform the corresponding unmap. The vcpu_put() still need to unconditionally unmap the current VNCR to close another ugly race.

Finally, the assignment of vncr_tlb::cpu is moved to be kept in sync with the actual mapping, similar to L1_VNCR_MAPPED being set.

Fixes 7270cc9157f47 ("KVM: arm64: nv: Handle VNCR_EL2 invalidation from MMU notifiers")

Reported-by [email protected]

Link https://lore.kernel.org/r/[email protected] open here

Signed-off-by Marc Zyngier <[email protected]>

Cc [email protected]

Reviewed-by Yuan Yao <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Oliver Upton <[email protected]>

arch/arm64/kvm/nested.c +32 ~~-10~~

index 8a602d074dbb..cf0d45059edb 100644
@@ -27,7 +27,7 @@ struct vncr_tlb {
 	bool			hpa_writable;
 
 	/* -1 when not mapped on a CPU */
-	int			cpu;
+	atomic_t		cpu;
 
 	/*
 	 * true if the TLB is valid. Can only be changed with the

@@ -894,16 +894,40 @@ void kvm_vcpu_load_hw_mmu(struct kvm_vcpu *vcpu)
 	}
 }
 
+/*
+ * Unmapping an L1 VNCR can happen concurrently without the mmu lock being
+ * effective (vcpu_put() vs TLBI handling). The atomic_xchg below ensures
+ * that only one CPU sets it to -1 while getting a valid CPU number back.
+ */
+static int unmap_l1_vncr(struct vncr_tlb *vt)
+{
+	int cpu = atomic_xchg_relaxed(&vt->cpu, -1);
+
+	if (cpu != -1)
+		clear_fixmap(vncr_fixmap(cpu));
+
+	return cpu;
+}
+
 static void this_cpu_reset_vncr_fixmap(struct kvm_vcpu *vcpu)
 {
 	if (!host_data_test_flag(L1_VNCR_MAPPED))
 		return;
 
-	BUG_ON(vcpu->arch.vncr_tlb->cpu != smp_processor_id());
 	BUG_ON(is_hyp_ctxt(vcpu));
 
-	clear_fixmap(vncr_fixmap(vcpu->arch.vncr_tlb->cpu));
-	vcpu->arch.vncr_tlb->cpu = -1;
+	/*
+	 * Unconditionally unmap the local VNCR if we have lost the race
+	 * against a concurrent TLBI. Otherwise we could end-up running
+	 * another vcpu with VNCR still mapped if the TLBI thread is
+	 * preempted between the exchange and the clear_fixmap().
+	 *
+	 * Note that we do not care about the TLBI nuking the fixmap behind
+	 * the back of an running vcpu. This will only generate a fault and
+	 * possibly a retranslation.
+	 */
+	if (unmap_l1_vncr(vcpu->arch.vncr_tlb) == -1)
+		clear_fixmap(vncr_fixmap(smp_processor_id()));
 	host_data_clear_flag(L1_VNCR_MAPPED);
 }

@@ -995,8 +1019,7 @@ u16 get_asid_by_regime(struct kvm_vcpu *vcpu, enum trans_regime regime)
 static void invalidate_vncr(struct vncr_tlb *vt)
 {
 	vt->valid = false;
-	if (vt->cpu != -1)
-		clear_fixmap(vncr_fixmap(vt->cpu));
+	unmap_l1_vncr(vt);
 }
 
 static bool vncr_tlb_intersects(struct vncr_tlb *vt, u64 addr,

@@ -1452,7 +1475,7 @@ static int kvm_translate_vncr(struct kvm_vcpu *vcpu, bool *is_gmem)
 		vt->hpa = pfn << PAGE_SHIFT;
 		vt->hpa_writable = writable;
 		vt->valid = true;
-		vt->cpu = -1;
+		atomic_set(&vt->cpu, -1);
 
 		kvm_make_request(KVM_REQ_MAP_L1_VNCR_EL2, vcpu);
 		kvm_release_faultin_page(vcpu->kvm, page, false, vt->wr.pw && vt->hpa_writable);

@@ -1583,8 +1606,6 @@ static void kvm_map_l1_vncr(struct kvm_vcpu *vcpu)
 	if (vt->wr.nG && get_asid_by_regime(vcpu, TR_EL20) != vt->wr.asid)
 		return;
 
-	vt->cpu = smp_processor_id();
-
 	if (vt->hpa_writable && vt->wr.pw && vt->wr.pr)
 		prot = PAGE_KERNEL;
 	else if (vt->wr.pr)

@@ -1599,7 +1620,8 @@ static void kvm_map_l1_vncr(struct kvm_vcpu *vcpu)
 	 * FIXME: WO doesn't work at all, need POE support in the kernel.
 	 */
 	if (pgprot_val(prot) != pgprot_val(PAGE_NONE)) {
-		__set_fixmap(vncr_fixmap(vt->cpu), vt->hpa, prot);
+		atomic_set(&vt->cpu, smp_processor_id());
+		__set_fixmap(vncr_fixmap(atomic_read(&vt->cpu)), vt->hpa, prot);
 		host_data_set_flag(L1_VNCR_MAPPED);
 	}
 }

Sasha Levin

maintainer

2026-09-11 10:59 UTC #1 lore raw

PATCH 1/2 KVM: arm64: nv: Fully update VNCR fixmap state in kvm_translate_vncr()

From: Oliver Upton <[email protected]>

[ Upstream commit 5949004d7032767e8fde1e8c986a33f241b2a192 ]

kvm_translate_vncr() first invalidates the pseudo-TLB entry and corresponding fixmap in anticipation of installing a new translation.

While the fixmap invalidation does clear the mapping from host stage-1, it does not clear the L1_VNCR_MAPPED flag. Depending on the state of the VNCR TLB at vcpu_put(), this could potentially precipitate a BUG_ON() if vt->cpu is reset.

Share a helper with kvm_vcpu_put_hw_mmu(), ensuring that KVM's view of the VNCR fixmap is in sync with the state of the VNCR TLB. Give it a slightly verbose name to make it obvious that it is meant to be used local to a CPU, unlike other VNCR TLB maintenance.

Fixes 069a05e535496 ("KVM: arm64: nv: Handle VNCR_EL2-triggered faults")

Signed-off-by Oliver Upton <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Marc Zyngier <[email protected]>

Stable-dep-of: 38640bc32be3 ("KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping")

Signed-off-by Sasha Levin <[email protected]>

---
 arch/arm64/kvm/nested.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

arch/arm64/kvm/nested.c +17 ~~-10~~

index b69df61f145a5..5a297c16bc338 100644
@@ -771,18 +771,24 @@ void kvm_vcpu_load_hw_mmu(struct kvm_vcpu *vcpu)
 	}
 }
 
+static void this_cpu_reset_vncr_fixmap(struct kvm_vcpu *vcpu)
+{
+	if (!host_data_test_flag(L1_VNCR_MAPPED))
+		return;
+
+	BUG_ON(vcpu->arch.vncr_tlb->cpu != smp_processor_id());
+	BUG_ON(is_hyp_ctxt(vcpu));
+
+	clear_fixmap(vncr_fixmap(vcpu->arch.vncr_tlb->cpu));
+	vcpu->arch.vncr_tlb->cpu = -1;
+	host_data_clear_flag(L1_VNCR_MAPPED);
+	atomic_dec(&vcpu->kvm->arch.vncr_map_count);
+}
+
 void kvm_vcpu_put_hw_mmu(struct kvm_vcpu *vcpu)
 {
 	/* Unconditionally drop the VNCR mapping if we have one */
-	if (host_data_test_flag(L1_VNCR_MAPPED)) {
-		BUG_ON(vcpu->arch.vncr_tlb->cpu != smp_processor_id());
-		BUG_ON(is_hyp_ctxt(vcpu));
-
-		clear_fixmap(vncr_fixmap(vcpu->arch.vncr_tlb->cpu));
-		vcpu->arch.vncr_tlb->cpu = -1;
-		host_data_clear_flag(L1_VNCR_MAPPED);
-		atomic_dec(&vcpu->kvm->arch.vncr_map_count);
-	}
+	this_cpu_reset_vncr_fixmap(vcpu);
 
 	/*
 	 * Keep a reference on the associated stage-2 MMU if the vCPU is

@@ -1244,7 +1250,8 @@ static int kvm_translate_vncr(struct kvm_vcpu *vcpu, bool *is_gmem)
 	 * We also prepare the next walk wilst we're at it.
 	 */
 	scoped_guard(write_lock, &vcpu->kvm->mmu_lock) {
-		invalidate_vncr(vt);
+		this_cpu_reset_vncr_fixmap(vcpu);
+		vt->valid = false;
 
 		vt->wi = (struct s1_walk_info) {
 			.regime	= TR_EL20,

Signature · 1 line

2.53.0

Sasha Levin

maintainer

2026-09-11 10:59 UTC #2 lore raw

PATCH 2/2 KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping

From: Marc Zyngier <[email protected]>

[ Upstream commit 38640bc32be3fcf9526d477155bc19d3f146231f ]

While VNCR TLB invalidation always occurs under the MMU lock, vcpu_put() doesn't, while it unmaps the VNCR page.

These two obviously can race, with a TOCTOU pattern on the TLBI path, and a BUG_ON() on the vcpu_put() path. And the two can end-up calling vncr_fixmap(-1), with extra lethal effects.

Finally, the assignment of vncr_tlb::cpu is moved to be kept in sync with the actual mapping, similar to L1_VNCR_MAPPED being set.

Fixes 7270cc9157f47 ("KVM: arm64: nv: Handle VNCR_EL2 invalidation from MMU notifiers")

Reported-by [email protected]

Link https://lore.kernel.org/r/[email protected] open here

Signed-off-by Marc Zyngier <[email protected]>

Cc [email protected]

Reviewed-by Yuan Yao <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Oliver Upton <[email protected]>

Signed-off-by Sasha Levin <[email protected]>

---
 arch/arm64/kvm/nested.c | 42 +++++++++++++++++++++++++++++++----------
 1 file changed, 32 insertions(+), 10 deletions(-)

arch/arm64/kvm/nested.c +32 ~~-10~~

index 5a297c16bc338..747aad09a40ee 100644
@@ -27,7 +27,7 @@ struct vncr_tlb {
 	bool			hpa_writable;
 
 	/* -1 when not mapped on a CPU */
-	int			cpu;
+	atomic_t		cpu;
 
 	/*
 	 * true if the TLB is valid. Can only be changed with the

@@ -771,16 +771,40 @@ void kvm_vcpu_load_hw_mmu(struct kvm_vcpu *vcpu)
 	}
 }
 
+/*
+ * Unmapping an L1 VNCR can happen concurrently without the mmu lock being
+ * effective (vcpu_put() vs TLBI handling). The atomic_xchg below ensures
+ * that only one CPU sets it to -1 while getting a valid CPU number back.
+ */
+static int unmap_l1_vncr(struct vncr_tlb *vt)
+{
+	int cpu = atomic_xchg_relaxed(&vt->cpu, -1);
+
+	if (cpu != -1)
+		clear_fixmap(vncr_fixmap(cpu));
+
+	return cpu;
+}
+
 static void this_cpu_reset_vncr_fixmap(struct kvm_vcpu *vcpu)
 {
 	if (!host_data_test_flag(L1_VNCR_MAPPED))
 		return;
 
-	BUG_ON(vcpu->arch.vncr_tlb->cpu != smp_processor_id());
 	BUG_ON(is_hyp_ctxt(vcpu));
 
-	clear_fixmap(vncr_fixmap(vcpu->arch.vncr_tlb->cpu));
-	vcpu->arch.vncr_tlb->cpu = -1;
+	/*
+	 * Unconditionally unmap the local VNCR if we have lost the race
+	 * against a concurrent TLBI. Otherwise we could end-up running
+	 * another vcpu with VNCR still mapped if the TLBI thread is
+	 * preempted between the exchange and the clear_fixmap().
+	 *
+	 * Note that we do not care about the TLBI nuking the fixmap behind
+	 * the back of an running vcpu. This will only generate a fault and
+	 * possibly a retranslation.
+	 */
+	if (unmap_l1_vncr(vcpu->arch.vncr_tlb) == -1)
+		clear_fixmap(vncr_fixmap(smp_processor_id()));
 	host_data_clear_flag(L1_VNCR_MAPPED);
 	atomic_dec(&vcpu->kvm->arch.vncr_map_count);
 }

@@ -870,8 +894,7 @@ u16 get_asid_by_regime(struct kvm_vcpu *vcpu, enum trans_regime regime)
 static void invalidate_vncr(struct vncr_tlb *vt)
 {
 	vt->valid = false;
-	if (vt->cpu != -1)
-		clear_fixmap(vncr_fixmap(vt->cpu));
+	unmap_l1_vncr(vt);
 }
 
 static bool vncr_tlb_intersects(struct vncr_tlb *vt, u64 addr,

@@ -1321,7 +1344,7 @@ static int kvm_translate_vncr(struct kvm_vcpu *vcpu, bool *is_gmem)
 		vt->hpa = pfn << PAGE_SHIFT;
 		vt->hpa_writable = writable;
 		vt->valid = true;
-		vt->cpu = -1;
+		atomic_set(&vt->cpu, -1);
 
 		kvm_make_request(KVM_REQ_MAP_L1_VNCR_EL2, vcpu);
 		kvm_release_faultin_page(vcpu->kvm, page, false, vt->wr.pw && vt->hpa_writable);

@@ -1452,8 +1475,6 @@ static void kvm_map_l1_vncr(struct kvm_vcpu *vcpu)
 	if (vt->wr.nG && get_asid_by_regime(vcpu, TR_EL20) != vt->wr.asid)
 		return;
 
-	vt->cpu = smp_processor_id();
-
 	if (vt->hpa_writable && vt->wr.pw && vt->wr.pr)
 		prot = PAGE_KERNEL;
 	else if (vt->wr.pr)

@@ -1468,7 +1489,8 @@ static void kvm_map_l1_vncr(struct kvm_vcpu *vcpu)
 	 * FIXME: WO doesn't work at all, need POE support in the kernel.
 	 */
 	if (pgprot_val(prot) != pgprot_val(PAGE_NONE)) {
-		__set_fixmap(vncr_fixmap(vt->cpu), vt->hpa, prot);
+		atomic_set(&vt->cpu, smp_processor_id());
+		__set_fixmap(vncr_fixmap(atomic_read(&vt->cpu)), vt->hpa, prot);
 		host_data_set_flag(L1_VNCR_MAPPED);
 		atomic_inc(&vcpu->kvm->arch.vncr_map_count);
 	}

Signature · 1 line

2.53.0