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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: David Howells <dhowells@redhat.com>
Subject: Re: [PATCH] oom: __task_cred() need rcu_read_lock()
Cc: kosaki.motohiro@jp.fujitsu.com, torvalds@osdl.org,
        akpm@linux-foundation.org, paulmck@linux.vnet.ibm.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, rientjes@google.com,
        kamezawa.hiroyu@jp.fujitsu.com
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> From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> 
> dump_tasks() needs to hold the RCU read lock around its access of the target
> task's UID.  To this end it should use task_uid() as it only needs that one
> thing from the creds.
> 
> The fact that dump_tasks() holds tasklist_lock is insufficient to prevent the
> target process replacing its credentials on another CPU.
> 
> Then, this patch change to call rcu_read_lock() explicitly.
> 
> 
> 	===================================================
> 	[ INFO: suspicious rcu_dereference_check() usage. ]
> 	---------------------------------------------------
> 	mm/oom_kill.c:410 invoked rcu_dereference_check() without protection!
> 
> 	other info that might help us debug this:
> 
> 	rcu_scheduler_active = 1, debug_locks = 1
> 	4 locks held by kworker/1:2/651:
> 	 #0:  (events){+.+.+.}, at: [<ffffffff8106aae7>]
> 	process_one_work+0x137/0x4a0
> 	 #1:  (moom_work){+.+...}, at: [<ffffffff8106aae7>]
> 	process_one_work+0x137/0x4a0
> 	 #2:  (tasklist_lock){.+.+..}, at: [<ffffffff810fafd4>]
> 	out_of_memory+0x164/0x3f0
> 	 #3:  (&(&p->alloc_lock)->rlock){+.+...}, at: [<ffffffff810fa48e>]
> 	find_lock_task_mm+0x2e/0x70
> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
> 
>  mm/oom_kill.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 5014e50..7b03102 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -372,7 +372,7 @@ static void dump_tasks(const struct mem_cgroup *mem)
>  		}
>  
>  		pr_info("[%5d] %5d %5d %8lu %8lu %3u     %3d         %5d %s\n",
> -			task->pid, __task_cred(task)->uid, task->tgid,
> +			task->pid, task_uid(task), task->tgid,
>  			task->mm->total_vm, get_mm_rss(task->mm),
>  			task_cpu(task), task->signal->oom_adj,
>  			task->signal->oom_score_adj, task->comm);

Thank you!



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