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SubjectRe: [PATCH] uids: Prevent tear down race
On Mon, Nov 02, 2009 at 12:09:40PM -0000, Thomas Gleixner wrote:
> 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>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Paul E. McKenney <paulmck@us.ibm.com>
> Cc: Kay Sievers <kay.sievers@vrfy.org>
> Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>

Acked-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>

> ---
> kernel/user.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Index: linux-2.6/kernel/user.c
> ===================================================================
> --- linux-2.6.orig/kernel/user.c
> +++ linux-2.6/kernel/user.c
> @@ -330,9 +330,9 @@ done:
> */
> static void free_user(struct user_struct *up, unsigned long flags)
> {
> - spin_unlock_irqrestore(&uidhash_lock, flags);
> INIT_DELAYED_WORK(&up->work, cleanup_user_struct);
> schedule_delayed_work(&up->work, msecs_to_jiffies(1000));
> + spin_unlock_irqrestore(&uidhash_lock, flags);
> }
>
> #else /* CONFIG_USER_SCHED && CONFIG_SYSFS */
>

--
regards,
Dhaval


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