On Wed, May 20, 2020 at 09:15:57PM +0100, Ben Hutchings wrote:

On Wed, 2020-05-20 at 21:10 +0100, Ben Hutchings wrote:

...

Please queue up the attached backport of commit 2351c88f8296 "watchdog: Fix the race between the release of watchdog_core_data and cdev" for 4.14.

And here's the corresponding version for 4.9.

Ben.

-- Ben Hutchings, Software Developer Codethink Ltd https://www.codethink.co.uk/ Dale House, 35 Dale Street Manchester, M1 2HF, United Kingdom

From 1cf1b24c844a037da38e6096a865bcab75aa05eb Mon Sep 17 00:00:00 2001 From: Kevin Hao haokexin@gmail.com Date: Tue, 8 Oct 2019 19:29:34 +0800 Subject: watchdog: Fix the race between the release of watchdog_core_data and cdev

commit 72139dfa2464e43957d330266994740bb7be2535 upstream.

The struct cdev is embedded in the struct watchdog_core_data. In the current code, we manage the watchdog_core_data with a kref, but the cdev is manged by a kobject. There is no any relationship between this kref and kobject. So it is possible that the watchdog_core_data is freed before the cdev is entirely released. We can easily get the following call trace with CONFIG_DEBUG_KOBJECT_RELEASE and CONFIG_DEBUG_OBJECTS_TIMERS enabled. ODEBUG: free active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x38 WARNING: CPU: 23 PID: 1028 at lib/debugobjects.c:481 debug_print_object+0xb0/0xf0 Modules linked in: softdog(-) deflate ctr twofish_generic twofish_common camellia_generic serpent_generic blowfish_generic blowfish_common cast5_generic cast_common cmac xcbc af_key sch_fq_codel openvswitch nsh nf_conncount nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 CPU: 23 PID: 1028 Comm: modprobe Not tainted 5.3.0-next-20190924-yoctodev-standard+ #180 Hardware name: Marvell OcteonTX CN96XX board (DT) pstate: 00400009 (nzcv daif +PAN -UAO) pc : debug_print_object+0xb0/0xf0 lr : debug_print_object+0xb0/0xf0 sp : ffff80001cbcfc70 x29: ffff80001cbcfc70 x28: ffff800010ea2128 x27: ffff800010bad000 x26: 0000000000000000 x25: ffff80001103c640 x24: ffff80001107b268 x23: ffff800010bad9e8 x22: ffff800010ea2128 x21: ffff000bc2c62af8 x20: ffff80001103c600 x19: ffff800010e867d8 x18: 0000000000000060 x17: 0000000000000000 x16: 0000000000000000 x15: ffff000bd7240470 x14: 6e6968207473696c x13: 5f72656d6974203a x12: 6570797420746365 x11: 6a626f2029302065 x10: 7461747320657669 x9 : 7463612820657669 x8 : 3378302f3078302b x7 : 0000000000001d7a x6 : ffff800010fd5889 x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000 x2 : ffff000bff948548 x1 : 276a1c9e1edc2300 x0 : 0000000000000000 Call trace: debug_print_object+0xb0/0xf0 debug_check_no_obj_freed+0x1e8/0x210 kfree+0x1b8/0x368 watchdog_cdev_unregister+0x88/0xc8 watchdog_dev_unregister+0x38/0x48 watchdog_unregister_device+0xa8/0x100 softdog_exit+0x18/0xfec4 [softdog] __arm64_sys_delete_module+0x174/0x200 el0_svc_handler+0xd0/0x1c8 el0_svc+0x8/0xc

This is a common issue when using cdev embedded in a struct. Fortunately, we already have a mechanism to solve this kind of issue. Please see commit 233ed09d7fda ("chardev: add helper function to register char devs with a struct device") for more detail.

Wait, 233ed09d7fda ("chardev: add helper function to register char devs with a struct device") only showed up in 4.12, it's not in 4.9, so how is this needed for 4.9?

thanks,

greg k-h