FAILED: patch "[PATCH] KVM: arm64: Pin the host vCPU before adjusting its PC under pKVM" failed to apply to 6.18-stable tree

[Sasha Levin <sashal@kernel.org>]

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 <stable@vger.kernel.org>.

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 a2bb5f5c67a95ac97fcd6859dab51be421cf26fe
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107050808-failed-6.18-a2bb5f5c67a9@kernel.org' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Thanks,
Sasha

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

From a2bb5f5c67a95ac97fcd6859dab51be421cf26fe Mon Sep 17 00:00:00 2001
From: Fuad Tabba <fuad.tabba@linux.dev>
Date: Thu, 1 Oct 2026 14:56:56 +0100
Subject: [PATCH] KVM: arm64: Pin the host vCPU before adjusting its PC under
 pKVM

Under pKVM, a page the host shares with EL2 is mapped at EL2 only while
it's pinned, and the host vCPU is pinned at its first KVM_RUN.
KVM_SET_VCPU_EVENTS with ext_dabt_pending reaches __kvm_adjust_pc() at
EL2 before that, and dereferencing the unmapped host vCPU panics the
hypervisor. Any process holding /dev/kvm on a pKVM host can trigger it.

Pin the host vCPU around the adjustment when no hyp vCPU is loaded for
it. A loaded hyp vCPU implies it's pinned. A pin fails only for memory
the host isn't sharing, and the request is then dropped like any other
bad host pointer.

Fixes: efa1368ba9f4b ("KVM: arm64: Commit exceptions from KVM_SET_VCPU_EVENTS immediately")
Reviewed-by: Vincent Donnefort <vdonnefort@google.com>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20261001135711.1640520-4-fuad.tabba@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/hyp/nvhe/hyp-main.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index b6bfe502bcd04..8b9d76e5336bb 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -546,8 +546,27 @@ static void handle___pkvm_host_mkyoung_guest(struct kvm_cpu_context *host_ctxt)
 static void handle___kvm_adjust_pc(struct kvm_cpu_context *host_ctxt)
 {
 	DECLARE_REG(struct kvm_vcpu *, vcpu, host_ctxt, 1);
+	struct pkvm_hyp_vcpu *hyp_vcpu;
+	struct kvm_vcpu *host_vcpu;
+
+	host_vcpu = __get_host_hyp_vcpus(vcpu, &hyp_vcpu);
+	if (host_vcpu) {
+		__kvm_adjust_pc(host_vcpu);
+		return;
+	}
+
+	/*
+	 * With no hyp vCPU loaded for it, the host vCPU may be unpinned,
+	 * and so unmapped at EL2: its first run pins it. A pin fails only
+	 * for memory the host isn't sharing, a bad pointer, so the request
+	 * is dropped.
+	 */
+	host_vcpu = kern_hyp_va(vcpu);
+	if (hyp_pin_shared_mem(host_vcpu, host_vcpu + 1))
+		return;
 
-	__kvm_adjust_pc(kern_hyp_va(vcpu));
+	__kvm_adjust_pc(host_vcpu);
+	hyp_unpin_shared_mem(host_vcpu, host_vcpu + 1);
 }
 
 static void handle___kvm_flush_vm_context(struct kvm_cpu_context *host_ctxt)
-- 
2.53.0