[PATCH v2] KVM: arm64: vgic-its: reject restored DTE with out-of-range num_eventid_bits

[Michael Bommarito]

Userspace can restore an ITS Device Table Entry whose Size field encodes
more EventID bits than the virtual ITS supports.  The live MAPD path
rejects that state, but vgic_its_restore_dte() accepts it and stores the
out-of-range value in dev->num_eventid_bits.

Reject restored DTEs with num_eventid_bits > VITS_TYPER_IDBITS before
allocating the device.  This mirrors the MAPD check and prevents the
restored state from reaching vgic_its_restore_itt(), where the unchecked
value can be converted into an oversized scan_its_table() range.

Fixes: 57a9a117154c ("KVM: arm64: vgic-its: Device table save/restore")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito at gmail.com>
---
Changes in v2:
- Trim the commit message to the root cause and fix.
- Shorten the in-code comment as suggested by Marc.
- Keep the validation logic unchanged.

 arch/arm64/kvm/vgic/vgic-its.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 2ea9f1c7ebcd0..1d7e5d560af4c 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -2307,6 +2307,10 @@ static int vgic_its_restore_dte(struct vgic_its *its, u32 id,
 	/* dte entry is valid */
 	offset = (entry & KVM_ITS_DTE_NEXT_MASK) >> KVM_ITS_DTE_NEXT_SHIFT;
 
+	/* Mimic the MAPD behaviour and reject invalid EID bits. */
+	if (num_eventid_bits > VITS_TYPER_IDBITS)
+		return -EINVAL;
+
 	if (!vgic_its_check_id(its, baser, id, NULL))
 		return -EINVAL;
 
-- 
2.53.0