[PATCH v3 06/13] KVM: x86/xen: Enforce 4-byte alignment of vcpu_info registration

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

Xen's map_guest_area() enforces that vcpu_info is aligned to
sizeof(xen_ulong_t). KVM has no such check, allowing a guest to
register vcpu_info at an arbitrary byte alignment.

Enforce unconditional 4-byte alignment regardless of the current
shinfo mode. This is sufficient because subsequent commits ensure
that all locked atomic operations on vcpu_info fields use at most
32-bit accesses. Return -ENXIO on failure, matching Xen's
map_guest_area() behaviour for unaligned requests.

Originally observed in review of an unrelated patch:
https://lore.kernel.org/all/20260604193554.1BA311F00893@smtp.kernel.org/

Cc: stable@vger.kernel.org
Fixes: 73e69a86347a ("KVM: x86/xen: register vcpu info")
Reported-by: sashiko-bot@kernel.org
Closes: https://lore.kernel.org/all/20260604193554.1BA311F00893@smtp.kernel.org
Assisted-by: Kiro:claude-opus-4.6-1m
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/kvm/xen.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 5eb23127e74e..941099ed50c9 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -941,6 +941,10 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 				break;
 			}
 
+			r = -ENXIO;
+			if (!IS_ALIGNED(data->u.gpa, sizeof(u32)))
+				break;
+
 			r = kvm_gpc_activate(&vcpu->arch.xen.vcpu_info_cache,
 					     data->u.gpa, sizeof(struct vcpu_info));
 		} else {
@@ -950,6 +954,10 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 				break;
 			}
 
+			r = -ENXIO;
+			if (!IS_ALIGNED(data->u.hva, sizeof(u32)))
+				break;
+
 			r = kvm_gpc_activate_hva(&vcpu->arch.xen.vcpu_info_cache,
 						 data->u.hva, sizeof(struct vcpu_info));
 		}
-- 
2.55.0