linux-kernel - Re: [2.6.37-rc1] sys_ioprio

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Date:	Sun, 7 Nov 2010 10:54:33 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Daniel J Blueman <daniel.blueman@...il.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>, jaxboe@...ionio.com
Subject: Re: [2.6.37-rc1] sys_ioprio_set and RCU locking...

On Tue, Nov 02, 2010 at 12:15:30PM +0000, Daniel J Blueman wrote:
> With 2.6.37-rc1, I observe sys_ioprio_set not taking the RCU lock [1]
> across access to the task credentials.
> 
> Inspecting the code in fs/ioprio.c, the tasklist_lock is held for read
> across the __task_cred call, which is presumably sufficient to prevent
> the task credentials becoming stale.
> 
> Thus, is there preference to take the RCU lock for read across the
> credential access eg at [2], or annotate the call?
> 
> Thanks,
>   Daniel
> 
> --- [1]
> 
> ===================================================
> 
> [ INFO: suspicious rcu_dereference_check() usage. ]
> 
> ---------------------------------------------------
> 
> kernel/pid.c:419 invoked rcu_dereference_check() without protection!
> 
> 
> 
> other info that might help us debug this:
> 
> 
> 
> 
> rcu_scheduler_active = 1, debug_locks = 1
> 
> 1 lock held by start-stop-daem/2246:
> 
>  #0:  (tasklist_lock){.?.?..}, at: [<ffffffff811a2dfa>]
> sys_ioprio_set+0x8a/0x400
> 
> 
> 
> stack backtrace:
> 
> Pid: 2246, comm: start-stop-daem Not tainted 2.6.37-rc1-330cd+ #2
> 
> Call Trace:
> 
>  [<ffffffff8109f5f4>] lockdep_rcu_dereference+0xa4/0xc0
> 
>  [<ffffffff81085651>] find_task_by_pid_ns+0x81/0x90
> 
>  [<ffffffff8108567d>] find_task_by_vpid+0x1d/0x20
> 
>  [<ffffffff811a3160>] sys_ioprio_set+0x3f0/0x400
> 
>  [<ffffffff816efa79>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> 
>  [<ffffffff81003482>] system_call_fastpath+0x16/0x1b
> 
> 
> --- [2]
> 
> Take the RCU lock for read across acquiring the pointer to the task
> credentials and dereferencing it.

Jens, does this look sane?

							Thanx, Paul

> Signed-off-by: Daniel J Blueman <daniel.blueman@...il.com>
> 
> diff --git a/fs/ioprio.c b/fs/ioprio.c
> index 748cfb9..00cc0e5 100644
> --- a/fs/ioprio.c
> +++ b/fs/ioprio.c
> @@ -139,8 +139,10 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who,
> int, ioprio)
>  				break;
> 
>  			do_each_thread(g, p) {
> +				rcu_read_lock();
>  				if (__task_cred(p)->uid != who)
>  					continue;
> +				rcu_read_unlock();
>  				ret = set_task_ioprio(p, ioprio);
>  				if (ret)
>  					goto free_uid;
> @@ -232,8 +234,10 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
>  				break;
> 
>  			do_each_thread(g, p) {
> +				rcu_read_lock();
>  				if (__task_cred(p)->uid != user->uid)
>  					continue;
> +				rcu_read_unlock();
>  				tmpio = get_task_ioprio(p);
>  				if (tmpio < 0)
>  					continue;
> -- 
> Daniel J Blueman
> --
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