[PATCH 5.15.y] KVM: arm64: vgic-its: Don't dereference a NULL collection on ITT save

[Aaron Ahmed <aarnahmd@amazon.com>]

From: Fuad Tabba <fuad.tabba@linux.dev>

[ Upstream commit c6c156d931c33b92362383cf76f6d6e1291dcbfe ]

MAPC with V=0 drops ite->collection but leaves the ITE on the device's
ITT list, and vgic_its_save_ite() dereferences it unconditionally. A
guest that issues MAPD, MAPTI and then MAPC(V=0) therefore oopses the
host when the VMM issues KVM_DEV_ARM_ITS_SAVE_TABLES to migrate it.
That sequence is UNPREDICTABLE per the architecture, but KVM already
handles the resulting state in the translate, MOVI and DISCARD paths.

Save a zeroed entry, which vgic_its_restore_ite() reads back as
invalid. Skipping the ITE instead would leave the ITT slot holding
whatever is in guest memory, and restore rejects an entry naming a
collection the restored collection table does not have.

Fixes: eff484e0298da ("KVM: arm64: vgic-its: ITT save and restore")
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20260807104102.2410744-2-fuad.tabba@linux.dev
Signed-off-by: Oliver Upton <oupton@kernel.org>
[aarnahmd: Upstream vgic_its_write_entry_lock() takes in the kind of table
 entry (ite) and looks up its size, while 5.15 takes in the size of an ite
 directly via ite_esz, because commit 3b2c81d5feb2 ("KVM: arm64: vgic-its:
 Add stronger type-checking to the ITS entry sizes") is not in 5.15.]
Signed-off-by: Aaron Ahmed <aarnahmd@amazon.com>
---
 arch/arm64/kvm/vgic/vgic-its.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 054aec8a72e35..e6ed23b14623f 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -2151,6 +2151,14 @@ static int vgic_its_save_ite(struct vgic_its *its, struct its_device *dev,
 	u32 next_offset;
 	u64 val;
 
+	/*
+	 * MAPC with V=0 keeps the ITEs mapped but drops their collection,
+	 * and with it the ICID. Save a zeroed entry, which the restore path
+	 * reads back as invalid.
+	 */
+	if (!ite->collection)
+		return vgic_its_write_entry_lock(its, gpa, 0ULL, ite_esz);
+
 	next_offset = compute_next_eventid_offset(&dev->itt_head, ite);
 	val = ((u64)next_offset << KVM_ITS_ITE_NEXT_SHIFT) |
 	       ((u64)ite->irq->intid << KVM_ITS_ITE_PINTID_SHIFT) |
-- 
2.47.3