[PATCH] static_call: fix null-ptr-deref in static_call_del_module

[Wang Hai <wanghai38@huawei.com>]

I got a NULL pointer dereference report when doing fault injection test:

BUG: kernel NULL pointer dereference, address: 0000000000000009
...
RIP: 0010:static_call_del_module+0x7a/0x100
...
Call Trace:
 static_call_module_notify+0x1e1/0x200
 notifier_call_chain_robust+0x6f/0xe0
 blocking_notifier_call_chain_robust+0x4e/0x70
 load_module+0x21f7/0x2b60
 __do_sys_finit_module+0xb0/0xf0
 ? __do_sys_finit_module+0xb0/0xf0
 __x64_sys_finit_module+0x1a/0x20
 do_syscall_64+0x34/0xb0
 entry_SYSCALL_64_after_hwframe+0x44/0xae

When loading a module, if it fails to allocate memory for static
calls, it will delete the non-existent mods (mods == 1) in the
static_call_module_notify()'s error path.

static_call_module_notify
	static_call_add_module
		__static_call_init
			site_mod = kzalloc() // fault injection
	static_call_del_module // access non-existent mods

This patch fixes the bug by skipping the operation when the key
has no mods.

Fixes: a945c8345ec0 ("static_call: Allow early init")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
---
 kernel/static_call.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/static_call.c b/kernel/static_call.c
index 43ba0b1e0edb..c3f8ffc5a52f 100644
--- a/kernel/static_call.c
+++ b/kernel/static_call.c
@@ -400,7 +400,7 @@ static void static_call_del_module(struct module *mod)
 
 	for (site = start; site < stop; site++) {
 		key = static_call_key(site);
-		if (key == prev_key)
+		if (key == prev_key || !static_call_key_has_mods(key))
 			continue;
 
 		prev_key = key;
-- 
2.17.1