uids: Prevent tear down race
authorThomas Gleixner <tglx@linutronix.de>
Mon, 2 Nov 2009 12:01:56 +0000 (13:01 +0100)
committerGreg Kroah-Hartman <gregkh@suse.de>
Tue, 8 Dec 2009 18:21:16 +0000 (10:21 -0800)
commit02caa6be73dbeacafbfa98ecaa39b1dab7d84eb6
treef8a9edad516bbfff239a17e7376dca9dca4776b3
parent7b9acdf264761c1a8fdd3696a04e3a47d3a44e23
uids: Prevent tear down race

commit b00bc0b237055b4c45816325ee14f0bd83e6f590 upstream.

Ingo triggered the following warning:

WARNING: at lib/debugobjects.c:255 debug_print_object+0x42/0x50()
Hardware name: System Product Name
ODEBUG: init active object type: timer_list
Modules linked in:
Pid: 2619, comm: dmesg Tainted: G        W  2.6.32-rc5-tip+ #5298
Call Trace:
 [<81035443>] warn_slowpath_common+0x6a/0x81
 [<8120e483>] ? debug_print_object+0x42/0x50
 [<81035498>] warn_slowpath_fmt+0x29/0x2c
 [<8120e483>] debug_print_object+0x42/0x50
 [<8120ec2a>] __debug_object_init+0x279/0x2d7
 [<8120ecb3>] debug_object_init+0x13/0x18
 [<810409d2>] init_timer_key+0x17/0x6f
 [<81041526>] free_uid+0x50/0x6c
 [<8104ed2d>] put_cred_rcu+0x61/0x72
 [<81067fac>] rcu_do_batch+0x70/0x121

debugobjects warns about an enqueued timer being initialized. If
CONFIG_USER_SCHED=y the user management code uses delayed work to
remove the user from the hash table and tear down the sysfs objects.

free_uid is called from RCU and initializes/schedules delayed work if
the usage count of the user_struct is 0. The init/schedule happens
outside of the uidhash_lock protected region which allows a concurrent
caller of find_user() to reference the about to be destroyed
user_struct w/o preventing the work from being scheduled. If the next
free_uid call happens before the work timer expired then the active
timer is initialized and the work scheduled again.

The race was introduced in commit 5cb350ba (sched: group scheduling,
sysfs tunables) and made more prominent by commit 3959214f (sched:
delayed cleanup of user_struct)

Move the init/schedule_delayed_work inside of the uidhash_lock
protected region to prevent the race.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@us.ibm.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
kernel/user.c