genirq/irqdesc: Prevent use-after-free in irq_find_at_or_after()

irq_find_at_or_after() dereferences the interrupt descriptor which is
returned by mt_find() while neither holding sparse_irq_lock nor RCU read
lock, which means the descriptor can be freed between mt_find() and the
dereference:

    CPU0                            CPU1
    desc = mt_find()
                                    delayed_free_desc(desc)
    irq_desc_get_irq(desc)

The use-after-free is reported by KASAN:

    Call trace:
     irq_get_next_irq+0x58/0x84
     show_stat+0x638/0x824
     seq_read_iter+0x158/0x4ec
     proc_reg_read_iter+0x94/0x12c
     vfs_read+0x1e0/0x2c8

    Freed by task 4471:
     slab_free_freelist_hook+0x174/0x1e0
     __kmem_cache_free+0xa4/0x1dc
     kfree+0x64/0x128
     irq_kobj_release+0x28/0x3c
     kobject_put+0xcc/0x1e0
     delayed_free_desc+0x14/0x2c
     rcu_do_batch+0x214/0x720

Guard the access with a RCU read lock section.

Fixes: 721255b ("genirq: Use a maple tree for interrupt descriptor management")
Signed-off-by: dicken.ding <dicken.ding@mediatek.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240524091739.31611-1-dicken.ding@mediatek.com

kernel/irq/irqdesc.c

@@ -160,7 +160,10 @@ static int irq_find_free_area(unsigned int from, unsigned int cnt)
 static unsigned int irq_find_at_or_after(unsigned int offset)
 {
 	unsigned long index = offset;
-	struct irq_desc *desc = mt_find(&sparse_irqs, &index, nr_irqs);
+	struct irq_desc *desc;
+
+	guard(rcu)();
+	desc = mt_find(&sparse_irqs, &index, nr_irqs);
 
 	return desc ? irq_desc_get_irq(desc) : nr_irqs;
 }