[PATCH v2 0/2] KVM: arm64: ID register finalisation fixes

[PATCH v2 0/2] KVM: arm64: ID register finalisation fixes

[Mark Brown]

While looking at some feature enablement I noticed that there are some
scenarios where we can end up with an inconsistently configured vCPU due
to finalizing configuration based on ID registers before we have blocked
writes to the ID registers. This series aims to clean up these issues.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
Changes in v2:
- Also replace the kvm_vm_has_ran_once() usage in kvm_set_vm_id_rg() and
  kvm_vm_finalize_sys_regs() with kvm_id_regs_final().
- Link to v1: https://patch.msgid.link/20260731-kvm-arm64-idreg-final-v1-0-3de2a5616dc6@kernel.org

---
Mark Brown (2):
      KVM: arm64: Finalize guest-wide sysregs prior to per-vCPU sysregs
      KVM: arm64: Block ID register changes after we rely on the values

 arch/arm64/include/asm/kvm_host.h |  8 ++++++
 arch/arm64/kvm/arm.c              |  2 +-
 arch/arm64/kvm/sys_regs.c         | 57 ++++++++++++++++++++++++---------------
 arch/arm64/kvm/sys_regs.h         |  2 +-
 arch/arm64/kvm/vgic/vgic-init.c   |  6 ++---
 5 files changed, 48 insertions(+), 27 deletions(-)
---
base-commit: f5098b6bae761e346ebcd9da7f95622c04733cff
change-id: 20260729-kvm-arm64-idreg-final-77cbc46b558a

Best regards,
--  
Mark Brown <broonie@kernel.org>

[PATCH v2 1/2] KVM: arm64: Finalize guest-wide sysregs prior to per-vCPU sysregs

[Mark Brown]

In commit d82d09d5ba4b ("KVM: arm64: Don't skip per-vcpu NV
initialisation") the NV register sanitisation was moved earlier in
kvm_finalize_sys_regs() so that it runs for each vCPU rather than only
once per guest. This means that for the first vCPU it runs prior to vGIC
finalization, but the vGIC finalization updates the ID registers which
the NV initialization uses so we may end up with a mismatch. For
example, HFGRTR_EL2.ICC_IGRPENn_EL1 depends on GICv3 being enabled in
ID_AA64PFR0_EL1.GIC so may be mistakenly marked or not marked as RES0.

Split the initialization which runs once per guest into a separate
function and run that before the per-vCPU initialisation for NV,
renaming the per-vCPU function to make it clear that it does per-vCPU
setup.

Fixes: d82d09d5ba4b ("KVM: arm64: Don't skip per-vcpu NV initialisation")
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/kvm/arm.c      |  2 +-
 arch/arm64/kvm/sys_regs.c | 40 ++++++++++++++++++++++++++--------------
 arch/arm64/kvm/sys_regs.h |  2 +-
 3 files changed, 28 insertions(+), 16 deletions(-)

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 50adfff75be8..2b75e1d5ca8d 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -931,7 +931,7 @@ int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
 			return ret;
 	}
 
-	ret = kvm_finalize_sys_regs(vcpu);
+	ret = kvm_vcpu_finalize_sys_regs(vcpu);
 	if (ret)
 		return ret;
 
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 5d5c579d4579..958d7ef78785 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -5755,25 +5755,14 @@ void kvm_calculate_traps(struct kvm_vcpu *vcpu)
 }
 
 /*
- * Perform last adjustments to the ID registers that are implied by the
+ * Do system register finalization that is shared by the whole guest. This
+ * includes last adjustments to the ID registers that are implied by the
  * configuration outside of the ID regs themselves, as well as any
  * initialisation that directly depend on these ID registers (such as
  * RES0/RES1 behaviours). This is not the place to configure traps though.
- *
- * Because this can be called once per CPU, changes must be idempotent.
  */
-int kvm_finalize_sys_regs(struct kvm_vcpu *vcpu)
+static int kvm_vm_finalize_sys_regs(struct kvm *kvm)
 {
-	struct kvm *kvm = vcpu->kvm;
-
-	guard(mutex)(&kvm->arch.config_lock);
-
-	if (vcpu_has_nv(vcpu)) {
-		int ret = kvm_init_nv_sysregs(vcpu);
-		if (ret)
-			return ret;
-	}
-
 	if (kvm_vm_has_ran_once(kvm))
 		return 0;
 
@@ -5825,6 +5814,29 @@ int kvm_finalize_sys_regs(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
+/*
+ * Because this can be called once per CPU, changes must be idempotent.
+ */
+int kvm_vcpu_finalize_sys_regs(struct kvm_vcpu *vcpu)
+{
+	struct kvm *kvm = vcpu->kvm;
+	int ret;
+
+	guard(mutex)(&kvm->arch.config_lock);
+
+	ret = kvm_vm_finalize_sys_regs(kvm);
+	if (ret)
+		return ret;
+
+	if (vcpu_has_nv(vcpu)) {
+		ret = kvm_init_nv_sysregs(vcpu);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
 int __init kvm_sys_reg_table_init(void)
 {
 	const struct sys_reg_desc *gicv3_regs;
diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
index 2a983664220c..402c5774d916 100644
--- a/arch/arm64/kvm/sys_regs.h
+++ b/arch/arm64/kvm/sys_regs.h
@@ -235,7 +235,7 @@ int kvm_sys_reg_set_user(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg,
 
 bool triage_sysreg_trap(struct kvm_vcpu *vcpu, int *sr_index);
 
-int kvm_finalize_sys_regs(struct kvm_vcpu *vcpu);
+int kvm_vcpu_finalize_sys_regs(struct kvm_vcpu *vcpu);
 
 #define AA32(_x)	.aarch32_map = AA32_##_x
 #define Op0(_x) 	.Op0 = _x

-- 
2.47.3

[PATCH v2 2/2] KVM: arm64: Block ID register changes after we rely on the values

[Mark Brown]

In commit c5bac1ef7df6b ("KVM: arm64: Move existing feature disabling
over to FGU infrastructure") a check was added to suppress duplicate
recalculation of FGUs based on a flag KVM_ARCH_FLAG_FGU_INITIALIZED. This
flag is set when we complete kvm_calculate_traps(), which is called from
kvm_arch_vcpu_run_pid_change(). There are several points where that
function could fail after we have calculated FGUs (eg, due to an invalid
timer configuration). If this happens then userspace will still be able
to write to the ID registers, writes to which are gated on
KVM_ARCH_FLAG_HAS_RAN_ONCE being set. This in turn means that the FGU
configuration for a running guest may not match the ID register
configuration.

This will result in issues based on the hypervisor assuming a consistent
configuration, for example it allows the creation of guests which have
untrapped access to system registers which are not context switched for
the guest.

A similar issue exists in kvm_init_nv_sysregs() where once sysreg_masks
is allocated the RES0/RES1 masks for registers are fixed based on the ID
register values at the time the function ran, and also for copying the
implementation ID registers to the hypervisor for pKVM.

There is a further issue with vGIC setup, creating a vGIC includes
updating the ID registers to reflect the GIC configuration. We refuse
to create a vGIC after the first vCPU has run but if a vCPU fails its
first run we may already have finalized the ID register values.

Avoid these issues by adding a new flag that we set when we finalize the
system registers, blocking ID register changes after that has been set
even if something fails later on. Do this in kvm_vm_finalize_sys_regs(),
this is where we finalize the GIC fields in the ID registers and happens
before we do the FGU and RES0/1 setup. A VMM which tries to create an
irqchip after failing to run a vCPU will now get -EBUSY rather than a
likely misconfigured guest. Userspace is not expected to try to run a
guest that fails to start, never mind try to repair the guest
configuration after doing so, so this is not expected to have any impact
on practical users.

Fixes: c5bac1ef7df6b ("KVM: arm64: Move existing feature disabling over to FGU infrastructure")
Fixes: 888f088070229 ("KVM: arm64: nv: Add sanitising to VNCR-backed sysregs")
Fixes: 03e1b89d051f ("KVM: arm64: Copy MIDR_EL1 into hyp VM when it is writable")
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/include/asm/kvm_host.h |  8 ++++++++
 arch/arm64/kvm/sys_regs.c         | 17 ++++++++++-------
 arch/arm64/kvm/vgic/vgic-init.c   |  6 ++----
 3 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index bae2c4f92ef5..8c8f7d83b6ff 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -367,6 +367,8 @@ struct kvm_arch {
 #define KVM_ARCH_FLAG_WRITABLE_IMP_ID_REGS		10
 	/* Unhandled SEAs are taken to userspace */
 #define KVM_ARCH_FLAG_EXIT_SEA				11
+	/* No further ID register changes possible */
+#define KVM_ARCH_FLAG_ID_REGS_FINAL			12
 	unsigned long flags;
 
 	/* VM-wide vCPU feature set */
@@ -1143,6 +1145,12 @@ struct kvm_vcpu_arch {
 #define vcpu_has_ptrauth(vcpu)		false
 #endif
 
+#define kvm_id_regs_final(kvm)						\
+	test_bit(KVM_ARCH_FLAG_ID_REGS_FINAL, &(kvm)->arch.flags)
+
+#define vcpu_id_regs_final(vcpu)					\
+	kvm_id_regs_final((vcpu)->kvm)
+
 #define vcpu_on_unsupported_cpu(vcpu)					\
 	vcpu_get_flag(vcpu, ON_UNSUPPORTED_CPU)
 
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 958d7ef78785..c8cfe30b56b4 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2427,9 +2427,10 @@ static int set_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 
 	/*
 	 * Once the VM has started the ID registers are immutable. Reject any
-	 * write that does not match the final register value.
+	 * write that does not match the final register value once we have
+	 * got far enough into first running the VM to use the values.
 	 */
-	if (kvm_vm_has_ran_once(vcpu->kvm)) {
+	if (vcpu_id_regs_final(vcpu)) {
 		if (val != read_id_reg(vcpu, rd))
 			ret = -EBUSY;
 		else
@@ -2463,7 +2464,7 @@ void kvm_set_vm_id_reg(struct kvm *kvm, u32 reg, u64 val)
 
 	lockdep_assert_held(&kvm->arch.config_lock);
 
-	if (KVM_BUG_ON(kvm_vm_has_ran_once(kvm) || !p, kvm))
+	if (KVM_BUG_ON(kvm_id_regs_final(kvm) || !p, kvm))
 		return;
 
 	*p = val;
@@ -3149,10 +3150,10 @@ static int set_imp_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
 		return -EINVAL;
 
 	/*
-	 * Once the VM has started the ID registers are immutable. Reject the
-	 * write if userspace tries to change it.
+	 * Once we have been far enough into starting the VM the ID registers
+	 * are immutable. Reject the write if userspace tries to change it.
 	 */
-	if (kvm_vm_has_ran_once(kvm))
+	if (kvm_id_regs_final(kvm))
 		return -EBUSY;
 
 	/*
@@ -5763,7 +5764,7 @@ void kvm_calculate_traps(struct kvm_vcpu *vcpu)
  */
 static int kvm_vm_finalize_sys_regs(struct kvm *kvm)
 {
-	if (kvm_vm_has_ran_once(kvm))
+	if (kvm_id_regs_final(kvm))
 		return 0;
 
 	/*
@@ -5811,6 +5812,8 @@ static int kvm_vm_finalize_sys_regs(struct kvm *kvm)
 		kvm_vgic_finalize_idregs(kvm);
 	}
 
+	set_bit(KVM_ARCH_FLAG_ID_REGS_FINAL, &kvm->arch.flags);
+
 	return 0;
 }
 
diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
index 907057881b26..4ffe0b7c3407 100644
--- a/arch/arm64/kvm/vgic/vgic-init.c
+++ b/arch/arm64/kvm/vgic/vgic-init.c
@@ -123,10 +123,8 @@ int kvm_vgic_create(struct kvm *kvm, u32 type)
 		goto out_unlock;
 	}
 
-	kvm_for_each_vcpu(i, vcpu, kvm) {
-		if (vcpu_has_run_once(vcpu))
-			goto out_unlock;
-	}
+	if (kvm_id_regs_final(kvm))
+		goto out_unlock;
 	ret = 0;
 
 	if (type == KVM_DEV_TYPE_ARM_VGIC_V2)

-- 
2.47.3

Re: [PATCH v2 0/2] KVM: arm64: ID register finalisation fixes

[Fuad Tabba]

Hi Mark,

On Mon, 3 Aug 2026 at 23:54, Mark Brown <broonie@kernel.org> wrote:
>
> While looking at some feature enablement I noticed that there are some
> scenarios where we can end up with an inconsistently configured vCPU due
> to finalizing configuration based on ID registers before we have blocked
> writes to the ID registers. This series aims to clean up these issues.
>
> Signed-off-by: Mark Brown <broonie@kernel.org>

I didn't want to hold this for the broader coverage you suggested, so
here is a focused test for the failed-first-run path [1]. It fails
without this series.

Tested on top of v7.2-rc5 under QEMU: boot tests, plus the
set_id_regs, vgic_init and get-reg-list selftests, the last also under
nested virt (-M vhe, kvm-arm.mode=nested).

For the series:

Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Tested-by: Fuad Tabba <fuad.tabba@linux.dev>

Cheers,
/fuad

[1] https://lore.kernel.org/all/20260804092421.1728880-1-fuad.tabba@linux.dev/

> ---
> Changes in v2:
> - Also replace the kvm_vm_has_ran_once() usage in kvm_set_vm_id_rg() and
>   kvm_vm_finalize_sys_regs() with kvm_id_regs_final().
> - Link to v1: https://patch.msgid.link/20260731-kvm-arm64-idreg-final-v1-0-3de2a5616dc6@kernel.org
>
> ---
> Mark Brown (2):
>       KVM: arm64: Finalize guest-wide sysregs prior to per-vCPU sysregs
>       KVM: arm64: Block ID register changes after we rely on the values
>
>  arch/arm64/include/asm/kvm_host.h |  8 ++++++
>  arch/arm64/kvm/arm.c              |  2 +-
>  arch/arm64/kvm/sys_regs.c         | 57 ++++++++++++++++++++++++---------------
>  arch/arm64/kvm/sys_regs.h         |  2 +-
>  arch/arm64/kvm/vgic/vgic-init.c   |  6 ++---
>  5 files changed, 48 insertions(+), 27 deletions(-)
> ---
> base-commit: f5098b6bae761e346ebcd9da7f95622c04733cff
> change-id: 20260729-kvm-arm64-idreg-final-77cbc46b558a
>
> Best regards,
> --
> Mark Brown <broonie@kernel.org>
>