irqchip/gic-v4: Don't allow a VMOVP on a dying VPE

Kunkun Jiang reported that there is a small window of opportunity for
userspace to force a change of affinity for a VPE while the VPE has already
been unmapped, but the corresponding doorbell interrupt still visible in
/proc/irq/.

Plug the race by checking the value of vmapp_count, which tracks whether
the VPE is mapped ot not, and returning an error in this case.

This involves making vmapp_count common to both GICv4.1 and its v4.0
ancestor.

Fixes: 64edfaa ("irqchip/gic-v4.1: Implement the v4.1 flavour of VMAPP")
Reported-by: Kunkun Jiang <jiangkunkun@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/c182ece6-2ba0-ce4f-3404-dba7a3ab6c52@huawei.com
Link: https://lore.kernel.org/all/20241002204959.2051709-1-maz@kernel.org

Author: Marc Zyngier

drivers/irqchip/irq-gic-v3-its.c

@@ -797,8 +797,8 @@ static struct its_vpe *its_build_vmapp_cmd(struct its_node *its,
 	its_encode_valid(cmd, desc->its_vmapp_cmd.valid);
 
 	if (!desc->its_vmapp_cmd.valid) {
+		alloc = !atomic_dec_return(&desc->its_vmapp_cmd.vpe->vmapp_count);
 		if (is_v4_1(its)) {
-			alloc = !atomic_dec_return(&desc->its_vmapp_cmd.vpe->vmapp_count);
 			its_encode_alloc(cmd, alloc);
 			/*
 			 * Unmapping a VPE is self-synchronizing on GICv4.1,
@@ -817,13 +817,13 @@ static struct its_vpe *its_build_vmapp_cmd(struct its_node *its,
 	its_encode_vpt_addr(cmd, vpt_addr);
 	its_encode_vpt_size(cmd, LPI_NRBITS - 1);
 
+	alloc = !atomic_fetch_inc(&desc->its_vmapp_cmd.vpe->vmapp_count);
+
 	if (!is_v4_1(its))
 		goto out;
 
 	vconf_addr = virt_to_phys(page_address(desc->its_vmapp_cmd.vpe->its_vm->vprop_page));
 
-	alloc = !atomic_fetch_inc(&desc->its_vmapp_cmd.vpe->vmapp_count);
-
 	its_encode_alloc(cmd, alloc);
 
 	/*
@@ -3806,6 +3806,13 @@ static int its_vpe_set_affinity(struct irq_data *d,
 	struct cpumask *table_mask;
 	unsigned long flags;
 
+	/*
+	 * Check if we're racing against a VPE being destroyed, for
+	 * which we don't want to allow a VMOVP.
+	 */
+	if (!atomic_read(&vpe->vmapp_count))
+		return -EINVAL;
+
 	/*
 	 * Changing affinity is mega expensive, so let's be as lazy as
 	 * we can and only do it if we really have to. Also, if mapped
@@ -4463,9 +4470,8 @@ static int its_vpe_init(struct its_vpe *vpe)
 	raw_spin_lock_init(&vpe->vpe_lock);
 	vpe->vpe_id = vpe_id;
 	vpe->vpt_page = vpt_page;
-	if (gic_rdists->has_rvpeid)
-		atomic_set(&vpe->vmapp_count, 0);
-	else
+	atomic_set(&vpe->vmapp_count, 0);
+	if (!gic_rdists->has_rvpeid)
 		vpe->vpe_proxy_event = -1;
 
 	return 0;

include/linux/irqchip/arm-gic-v4.h

@@ -66,10 +66,12 @@ struct its_vpe {
 				bool	enabled;
 				bool	group;
 			}			sgi_config[16];
-			atomic_t vmapp_count;
 		};
 	};
 
+	/* Track the VPE being mapped */
+	atomic_t vmapp_count;
+
 	/*
 	 * Ensures mutual exclusion between affinity setting of the
 	 * vPE and vLPI operations using vpe->col_idx.