rnbd-clt: fix refcount underflow in device unmap path #478
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During device unmapping (triggered by module unload or explicit unmap), a refcount underflow occurs causing a use-after-free warning: [14747.574913] ------------[ cut here ]------------ [14747.574916] refcount_t: underflow; use-after-free. [14747.574917] WARNING: lib/refcount.c:28 at refcount_warn_saturate+0x55/0x90, CPU#9: kworker/9:1/378 [14747.574924] Modules linked in: rnbd_client(-) rtrs_client rnbd_server rtrs_server rtrs_core ... [14747.574998] CPU: 9 UID: 0 PID: 378 Comm: kworker/9:1 Tainted: G O N 6.19.0-rc3lblk-fnext+ #42 PREEMPT(voluntary) [14747.575005] Workqueue: rnbd_clt_wq unmap_device_work [rnbd_client] [14747.575010] RIP: 0010:refcount_warn_saturate+0x55/0x90 [14747.575037] Call Trace: [14747.575038] <TASK> [14747.575038] rnbd_clt_unmap_device+0x170/0x1d0 [rnbd_client] [14747.575044] process_one_work+0x211/0x600 [14747.575052] worker_thread+0x184/0x330 [14747.575055] ? __pfx_worker_thread+0x10/0x10 [14747.575058] kthread+0x10d/0x250 [14747.575062] ? __pfx_kthread+0x10/0x10 [14747.575066] ret_from_fork+0x319/0x390 [14747.575069] ? __pfx_kthread+0x10/0x10 [14747.575072] ret_from_fork_asm+0x1a/0x30 [14747.575083] </TASK> [14747.575096] ---[ end trace 0000000000000000 ]--- Befor this patch :- The bug is a double kobject_put() on dev->kobj during device cleanup. Kobject Lifecycle: kobject_init_and_add() sets kobj.kref = 1 (initialization) kobject_put() sets kobj.kref = 0 (should be called once) * Before this patch: rnbd_clt_unmap_device() rnbd_destroy_sysfs() kobject_del(&dev->kobj) [remove from sysfs] kobject_put(&dev->kobj) PUT #1 (WRONG!) kref: 1 to 0 rnbd_dev_release() kfree(dev) [DEVICE FREED!] rnbd_destroy_gen_disk() [use-after-free!] rnbd_clt_put_dev() refcount_dec_and_test(&dev->refcount) kobject_put(&dev->kobj) PUT #2 (UNDERFLOW!) kref: 0 to -1 [WARNING!] The first kobject_put() in rnbd_destroy_sysfs() prematurely frees the device via rnbd_dev_release(), then the second kobject_put() in rnbd_clt_put_dev() causes refcount underflow. * After this patch :- Remove kobject_put() from rnbd_destroy_sysfs(). This function should only remove sysfs visibility (kobject_del), not manage object lifetime. Call Graph (FIXED): rnbd_clt_unmap_device() rnbd_destroy_sysfs() kobject_del(&dev->kobj) [remove from sysfs only] [kref unchanged: 1] rnbd_destroy_gen_disk() [device still valid] rnbd_clt_put_dev() refcount_dec_and_test(&dev->refcount) kobject_put(&dev->kobj) ONLY PUT (CORRECT!) kref: 1 to 0 [BALANCED] rnbd_dev_release() kfree(dev) [CLEAN DESTRUCTION] This follows the kernel pattern where sysfs removal (kobject_del) is separate from object destruction (kobject_put). Fixes: 581cf83 ("block: rnbd: add .release to rnbd_dev_ktype") Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com> Acked-by: Jack Wang <jinpu.wang@ionos.com> Reviewed-by: Jack Wang <jinpu.wang@ionos.com>
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Pull request for series with
subject: rnbd-clt: fix refcount underflow in device unmap path
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1041453