When linux is booted in EL1, macro "host_data_ptr()" is a wrapper that
resolves to "&per_cpu_ptr_nvhe_sym(kvm_host_data, cpu)",
is_hyp_mode_available() return false during kvm_arm_init, the per-CPU base
pointer __kvm_nvhe_kvm_arm_hyp_percpu_base[cpu] remains uninitialized.
Consequently, any access via per_cpu_ptr_nvhe_sym(kvm_host_data, cpu)
will result in a NULL pointer.

Add is_kvm_arm_initialised() condition check to ensure that kvm_arm_init
completes all necessary initialization steps, including init_hyp_mode.

Fixes: 054b88391bbe2 ("KVM: arm64: Support trace filtering for guests")
Signed-off-by: Yingchao Deng <yingchao.deng@oss.qualcomm.com>
Reviewed-by: James Clark <james.clark@linaro.org>
---
Add a check to prevent accessing uninitialized per-CPU data.
---
Changes in v2:
1. Move the warning to the end in order to improve readability. No
functional change intended
- Link to v1: https://lore.kernel.org/r/20250901-etm_crash-v1-1-ce65e44c137c@oss.qualcomm.com
---
 arch/arm64/kvm/debug.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
index 381382c19fe4741980c79b08bbdab6a1bcd825ad..593fcbbc7c014335c5999b774f9bfa367e709cb5 100644
--- a/arch/arm64/kvm/debug.c
+++ b/arch/arm64/kvm/debug.c
@@ -233,7 +233,7 @@ void kvm_debug_handle_oslar(struct kvm_vcpu *vcpu, u64 val)
 void kvm_enable_trbe(void)
 {
 	if (has_vhe() || is_protected_kvm_enabled() ||
-	    WARN_ON_ONCE(preemptible()))
+	    !is_kvm_arm_initialised() || WARN_ON_ONCE(preemptible()))
 		return;
 
 	host_data_set_flag(TRBE_ENABLED);
@@ -243,7 +243,7 @@ EXPORT_SYMBOL_GPL(kvm_enable_trbe);
 void kvm_disable_trbe(void)
 {
 	if (has_vhe() || is_protected_kvm_enabled() ||
-	    WARN_ON_ONCE(preemptible()))
+	    !is_kvm_arm_initialised() || WARN_ON_ONCE(preemptible()))
 		return;
 
 	host_data_clear_flag(TRBE_ENABLED);
@@ -252,7 +252,8 @@ EXPORT_SYMBOL_GPL(kvm_disable_trbe);
 
 void kvm_tracing_set_el1_configuration(u64 trfcr_while_in_guest)
 {
-	if (is_protected_kvm_enabled() || WARN_ON_ONCE(preemptible()))
+	if (is_protected_kvm_enabled() || !is_kvm_arm_initialised() ||
+	    WARN_ON_ONCE(preemptible()))
 		return;
 
 	if (has_vhe()) {

---
base-commit: 8cd53fb40a304576fa86ba985f3045d5c55b0ae3
change-id: 20250901-etm_crash-0ee923eee98c

Best regards,
-- 
Yingchao Deng <yingchao.deng@oss.qualcomm.com>

On Tue, 02 Sep 2025 11:48:25 +0800, Yingchao Deng wrote:
> When linux is booted in EL1, macro "host_data_ptr()" is a wrapper that
> resolves to "&per_cpu_ptr_nvhe_sym(kvm_host_data, cpu)",
> is_hyp_mode_available() return false during kvm_arm_init, the per-CPU base
> pointer __kvm_nvhe_kvm_arm_hyp_percpu_base[cpu] remains uninitialized.
> Consequently, any access via per_cpu_ptr_nvhe_sym(kvm_host_data, cpu)
> will result in a NULL pointer.
> 
> [...]

Applied to next, thanks!

[1/1] KVM: arm64: Fix NULL pointer access issue
      commit 27d2b47eef033f1fc6c0452dc1017e43dad5fe14

Cheers,

	M.
-- 
Without deviation from the norm, progress is not possible.

Hi Yingchao,

The shortlog is extremely vague, you should aim to succinctly describe
the functional change of your patch. e.g.

  KVM: arm64: Return early from trace helpers when KVM isn't available

On Tue, Sep 02, 2025 at 11:48:25AM +0800, Yingchao Deng wrote:
> When linux is booted in EL1, macro "host_data_ptr()" is a wrapper that
> resolves to "&per_cpu_ptr_nvhe_sym(kvm_host_data, cpu)",
> is_hyp_mode_available() return false during kvm_arm_init, the per-CPU base
> pointer __kvm_nvhe_kvm_arm_hyp_percpu_base[cpu] remains uninitialized.
> Consequently, any access via per_cpu_ptr_nvhe_sym(kvm_host_data, cpu)
> will result in a NULL pointer.
> 
> Add is_kvm_arm_initialised() condition check to ensure that kvm_arm_init
> completes all necessary initialization steps, including init_hyp_mode.

OTOH, the changelog is very mechanical and hard to grok.

  When linux is booted at EL1, host_data_ptr() resolves to the nVHE
  hypervisor's copy of host data. When hyp mode isn't available for
  KVM the nVHE percpu bases remain uninitialized. Consequently, any usage
  of host_data_ptr() will result in a NULL dereference which has been
  observed in KVM's trace filtering helpers.

  Add an early return to the trace filtering helpers if KVM isn't
  initialized, avoiding the NULL dereference.

> Fixes: 054b88391bbe2 ("KVM: arm64: Support trace filtering for guests")
> Signed-off-by: Yingchao Deng <yingchao.deng@oss.qualcomm.com>
> Reviewed-by: James Clark <james.clark@linaro.org>
> ---
> Add a check to prevent accessing uninitialized per-CPU data.
> ---
> Changes in v2:
> 1. Move the warning to the end in order to improve readability. No
> functional change intended

IMO, the warning should be the very first condition we evaluate. Even if
the system configuration leads to an early return anyway (e.g. protected
mode) the caller is not invoking these helpers from the right context.

Thanks,
Oliver

On Thu, 04 Sep 2025 08:26:15 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> Hi Yingchao,
> 
> The shortlog is extremely vague, you should aim to succinctly describe
> the functional change of your patch. e.g.
> 
>   KVM: arm64: Return early from trace helpers when KVM isn't available
> 
> On Tue, Sep 02, 2025 at 11:48:25AM +0800, Yingchao Deng wrote:
> > When linux is booted in EL1, macro "host_data_ptr()" is a wrapper that
> > resolves to "&per_cpu_ptr_nvhe_sym(kvm_host_data, cpu)",
> > is_hyp_mode_available() return false during kvm_arm_init, the per-CPU base
> > pointer __kvm_nvhe_kvm_arm_hyp_percpu_base[cpu] remains uninitialized.
> > Consequently, any access via per_cpu_ptr_nvhe_sym(kvm_host_data, cpu)
> > will result in a NULL pointer.
> > 
> > Add is_kvm_arm_initialised() condition check to ensure that kvm_arm_init
> > completes all necessary initialization steps, including init_hyp_mode.
> 
> OTOH, the changelog is very mechanical and hard to grok.
> 
>   When linux is booted at EL1, host_data_ptr() resolves to the nVHE
>   hypervisor's copy of host data. When hyp mode isn't available for
>   KVM the nVHE percpu bases remain uninitialized. Consequently, any usage
>   of host_data_ptr() will result in a NULL dereference which has been
>   observed in KVM's trace filtering helpers.
> 
>   Add an early return to the trace filtering helpers if KVM isn't
>   initialized, avoiding the NULL dereference.
> 
> > Fixes: 054b88391bbe2 ("KVM: arm64: Support trace filtering for guests")
> > Signed-off-by: Yingchao Deng <yingchao.deng@oss.qualcomm.com>
> > Reviewed-by: James Clark <james.clark@linaro.org>
> > ---
> > Add a check to prevent accessing uninitialized per-CPU data.
> > ---
> > Changes in v2:
> > 1. Move the warning to the end in order to improve readability. No
> > functional change intended
> 
> IMO, the warning should be the very first condition we evaluate. Even if
> the system configuration leads to an early return anyway (e.g. protected
> mode) the caller is not invoking these helpers from the right context.

This is what I intend to merge, with the commit log repainted as
above, and the helpers refactored in a slightly less ugly way (well,
at least more to my own taste, YMMV).

	M.

From 27d2b47eef033f1fc6c0452dc1017e43dad5fe14 Mon Sep 17 00:00:00 2001
 From: Yingchao Deng <yingchao.deng@oss.qualcomm.com>
Date: Tue, 2 Sep 2025 11:48:25 +0800
Subject: [PATCH] KVM: arm64: Return early from trace helpers when KVM isn't
 available

When Linux is booted at EL1, host_data_ptr() resolves to the nVHE
hypervisor's copy of host data. When hyp mode isn't available for
KVM the nVHE percpu bases remain uninitialized. Consequently, any usage
of host_data_ptr() will result in a NULL dereference which has been
observed in KVM's trace filtering helpers.

Add an early return to the trace filtering helpers if KVM isn't
initialized, avoiding the NULL dereference. Take this opportunity
to move the TRBE-skipping checks to a common helper.

Fixes: 054b88391bbe2 ("KVM: arm64: Support trace filtering for guests")
Signed-off-by: Yingchao Deng <yingchao.deng@oss.qualcomm.com>
Reviewed-by: James Clark <james.clark@linaro.org>
[maz: repainted the helpers to be readable, and the commit message
 with Oliver's suggestion]
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/debug.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
index 381382c19fe47..1aaeb40a9a388 100644
--- a/arch/arm64/kvm/debug.c
+++ b/arch/arm64/kvm/debug.c
@@ -230,29 +230,29 @@ void kvm_debug_handle_oslar(struct kvm_vcpu *vcpu, u64 val)
 	preempt_enable();
 }
 
-void kvm_enable_trbe(void)
+static bool skip_trbe_access(bool skip_condition)
 {
-	if (has_vhe() || is_protected_kvm_enabled() ||
-	    WARN_ON_ONCE(preemptible()))
-		return;
+	return (WARN_ON_ONCE(preemptible()) || skip_condition ||
+		is_protected_kvm_enabled() || !is_kvm_arm_initialised());
+}
 
-	host_data_set_flag(TRBE_ENABLED);
+void kvm_enable_trbe(void)
+{
+	if (!skip_trbe_access(has_vhe()))
+		host_data_set_flag(TRBE_ENABLED);
 }
 EXPORT_SYMBOL_GPL(kvm_enable_trbe);
 
 void kvm_disable_trbe(void)
 {
-	if (has_vhe() || is_protected_kvm_enabled() ||
-	    WARN_ON_ONCE(preemptible()))
-		return;
-
-	host_data_clear_flag(TRBE_ENABLED);
+	if (!skip_trbe_access(has_vhe()))
+		host_data_clear_flag(TRBE_ENABLED);
 }
 EXPORT_SYMBOL_GPL(kvm_disable_trbe);
 
 void kvm_tracing_set_el1_configuration(u64 trfcr_while_in_guest)
 {
-	if (is_protected_kvm_enabled() || WARN_ON_ONCE(preemptible()))
+	if (skip_trbe_access(false))
 		return;
 
 	if (has_vhe()) {
-- 
2.39.2


-- 
Without deviation from the norm, progress is not possible.