[PATCH 1/1] irq: Fix uaf issue in irq_find_at_or_after

[dicken.ding]

The function "irq_find_at_or_after" is at the risk of use-after-free
due to the race condition between the functions "delayer_free_desc"
and "irq_desc_get_irq". The function "delayer_free_desc" could be
called between "mt_find" and "irq_desc_get_irq" due to the absence
of any locks to ensure atomic operations on the "irq_desc" structure.

In this patch, we introduce a pair of locks, namely "rcu_read_lock"
and "rcu_read_unlock" to prevent the occurrence of use-after-free in
"irq_find_at_or_after".

Call trace:
 dump_backtrace+0xec/0x138
 show_stack+0x18/0x24
 dump_stack_lvl+0x50/0x6c
 print_report+0x1b0/0x714
 kasan_report+0xc4/0x124
 __do_kernel_fault+0xc0/0x368
 do_bad_area+0x30/0xdc
 do_tag_check_fault+0x20/0x34
 do_mem_abort+0x58/0x118
 el1_abort+0x3c/0x5c
 el1h_64_sync_handler+0x54/0x90
 el1h_64_sync+0x68/0x6c
 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
 __arm64_sys_pread64+0x84/0xcc
 invoke_syscall+0x58/0x114
 el0_svc_common+0x80/0xe0
 do_el0_svc+0x1c/0x28
 el0_svc+0x38/0x68
 el0t_64_sync_handler+0x68/0xbc
 el0t_64_sync+0x1a8/0x1ac

Freed by task 4471:
 kasan_save_stack+0x40/0x70
 save_stack_info+0x34/0x128
 kasan_save_free_info+0x18/0x28
 ____kasan_slab_free+0x254/0x25c
 __kasan_slab_free+0x10/0x20
 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
 rcu_core+0x1b0/0x408
 rcu_core_si+0x10/0x20
 __do_softirq+0x154/0x470

Signed-off-by: dicken.ding <dicken.ding at mediatek.com>
---
 kernel/irq/irqdesc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/irq/irqdesc.c b/kernel/irq/irqdesc.c
index 88ac3652fcf2..14124e1bcb03 100644
--- a/kernel/irq/irqdesc.c
+++ b/kernel/irq/irqdesc.c
@@ -160,9 +160,15 @@ 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;
+	unsigned int irq = nr_irqs;
+
+	rcu_read_lock();
 	struct irq_desc *desc = mt_find(&sparse_irqs, &index, nr_irqs);
+	if (desc)
+		irq = irq_desc_get_irq(desc);
+	rcu_read_unlock();
 
-	return desc ? irq_desc_get_irq(desc) : nr_irqs;
+	return irq;
 }
 
 static void irq_insert_desc(unsigned int irq, struct irq_desc *desc)
-- 
2.18.0