linux-kernel - Re: [PATCH v2] KVM: arm64: Fix NULL pointer access issue

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <86y0qgatgf.wl-maz@kernel.org>
Date: Mon, 15 Sep 2025 11:41:36 +0100
From: Marc Zyngier <maz@...nel.org>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: Yingchao Deng <yingchao.deng@....qualcomm.com>,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	James Clark <james.clark@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	quic_yingdeng@...cinc.com,
	jinlong.mao@....qualcomm.com,
	tingwei.zhang@....qualcomm.com
Subject: Re: [PATCH v2] KVM: arm64: Fix NULL pointer access issue

On Thu, 04 Sep 2025 08:26:15 +0100,
Oliver Upton <oliver.upton@...ux.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@....qualcomm.com>
> > Reviewed-by: James Clark <james.clark@...aro.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@....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@....qualcomm.com>
Reviewed-by: James Clark <james.clark@...aro.org>
[maz: repainted the helpers to be readable, and the commit message
 with Oliver's suggestion]
Signed-off-by: Marc Zyngier <maz@...nel.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.