linux-kernel - Re: [PATCH] exit: move exit_task_namespaces() after exit_task

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Date:   Fri, 15 Dec 2017 00:56:40 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] exit: move exit_task_namespaces() after exit_task_work()

Cong Wang <xiyou.wangcong@...il.com> writes:

> syzbot reported we have a use-after-free when mqueue_evict_inode()
> is called on __cleanup_mnt() path, where the ipc ns is already
> freed by the previous exit_task_namespaces(). We can just move
> it after after exit_task_work() to avoid this use-after-free.

How does that possibly work. (I haven't seen this syzbot report).

Looking at the code we have get_ns_from_inode.  Which takes the mq_lock,
sees if the pointer is NULL and takes a reference if it is non-NULL.

Meanwhile put_ipc_ns calls mq_clear_sbinfo(ns) with the mq_lock held
when the count drops to zero.

Where is the race in that?

The rest of mqueue_evict_inode uses the returned pointer and
tests that the pointer is non-NULL before user it.

So either szbot is giving you a bad report or there is a subtle race
there I am not seeing.  The change below is not at all the proper way to
fix a subtle race.

Eric


>
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: stable@...r.kernel.org
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  kernel/exit.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 6b4298a41167..909e43c45158 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -861,8 +861,8 @@ void __noreturn do_exit(long code)
>  	exit_fs(tsk);
>  	if (group_dead)
>  		disassociate_ctty(1);
> -	exit_task_namespaces(tsk);
>  	exit_task_work(tsk);
> +	exit_task_namespaces(tsk);
>  	exit_thread(tsk);
>  
>  	/*