linux-kernel - Re: [PATCH v2] Drop reference added by grab

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Date:   Fri, 06 Jan 2017 15:38:25 +1300
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Zhou Chengming <zhouchengming1@...wei.com>
Cc:     <linux-kernel@...r.kernel.org>, <viro@...iv.linux.org.uk>,
        <balbi@...nel.org>, <lizefan@...wei.com>,
        <konishi.ryusuke@....ntt.co.jp>, <jack@...e.cz>,
        <dmitry.torokhov@...il.com>, <caiqian@...hat.com>,
        <yangshukui@...wei.com>, <guohanjun@...wei.com>,
        <dingtianhong@...wei.com>
Subject: Re: [PATCH v2] Drop reference added by grab_header

Zhou Chengming <zhouchengming1@...wei.com> writes:

> Fixes CVE-2016-9191, proc_sys_readdir doesn't drop reference
> added by grab_header when return from !dir_emit_dots path.
> It can cause any path called unregister_sysctl_table will
> wait forever.

Applied.  Thank you.

I fixed up the subject line to be more specific:
"sysctl: Drop reference added by grab_header in proc_sys_readdir"

And added the tags:

Fixes: f0c3b5093add ("[readdir] convert procfs")
Cc: stable@...r.kernel.org

So that everywhere that is affected by this reference leak can get found
and fixed.

Eric



> The calltrace of CVE-2016-9191:
>
> [ 5535.960522] Call Trace:
> [ 5535.963265]  [<ffffffff817cdaaf>] schedule+0x3f/0xa0
> [ 5535.968817]  [<ffffffff817d33fb>] schedule_timeout+0x3db/0x6f0
> [ 5535.975346]  [<ffffffff817cf055>] ? wait_for_completion+0x45/0x130
> [ 5535.982256]  [<ffffffff817cf0d3>] wait_for_completion+0xc3/0x130
> [ 5535.988972]  [<ffffffff810d1fd0>] ? wake_up_q+0x80/0x80
> [ 5535.994804]  [<ffffffff8130de64>] drop_sysctl_table+0xc4/0xe0
> [ 5536.001227]  [<ffffffff8130de17>] drop_sysctl_table+0x77/0xe0
> [ 5536.007648]  [<ffffffff8130decd>] unregister_sysctl_table+0x4d/0xa0
> [ 5536.014654]  [<ffffffff8130deff>] unregister_sysctl_table+0x7f/0xa0
> [ 5536.021657]  [<ffffffff810f57f5>] unregister_sched_domain_sysctl+0x15/0x40
> [ 5536.029344]  [<ffffffff810d7704>] partition_sched_domains+0x44/0x450
> [ 5536.036447]  [<ffffffff817d0761>] ? __mutex_unlock_slowpath+0x111/0x1f0
> [ 5536.043844]  [<ffffffff81167684>] rebuild_sched_domains_locked+0x64/0xb0
> [ 5536.051336]  [<ffffffff8116789d>] update_flag+0x11d/0x210
> [ 5536.057373]  [<ffffffff817cf61f>] ? mutex_lock_nested+0x2df/0x450
> [ 5536.064186]  [<ffffffff81167acb>] ? cpuset_css_offline+0x1b/0x60
> [ 5536.070899]  [<ffffffff810fce3d>] ? trace_hardirqs_on+0xd/0x10
> [ 5536.077420]  [<ffffffff817cf61f>] ? mutex_lock_nested+0x2df/0x450
> [ 5536.084234]  [<ffffffff8115a9f5>] ? css_killed_work_fn+0x25/0x220
> [ 5536.091049]  [<ffffffff81167ae5>] cpuset_css_offline+0x35/0x60
> [ 5536.097571]  [<ffffffff8115aa2c>] css_killed_work_fn+0x5c/0x220
> [ 5536.104207]  [<ffffffff810bc83f>] process_one_work+0x1df/0x710
> [ 5536.110736]  [<ffffffff810bc7c0>] ? process_one_work+0x160/0x710
> [ 5536.117461]  [<ffffffff810bce9b>] worker_thread+0x12b/0x4a0
> [ 5536.123697]  [<ffffffff810bcd70>] ? process_one_work+0x710/0x710
> [ 5536.130426]  [<ffffffff810c3f7e>] kthread+0xfe/0x120
> [ 5536.135991]  [<ffffffff817d4baf>] ret_from_fork+0x1f/0x40
> [ 5536.142041]  [<ffffffff810c3e80>] ? kthread_create_on_node+0x230/0x230
>
> One cgroup maintainer mentioned that "cgroup is trying to offline
> a cpuset css, which takes place under cgroup_mutex.  The offlining
> ends up trying to drain active usages of a sysctl table which apprently
> is not happening."
> The real reason is that proc_sys_readdir doesn't drop reference added
> by grab_header when return from !dir_emit_dots path. So this cpuset
> offline path will wait here forever.
>
> See here for details: http://www.openwall.com/lists/oss-security/2016/11/04/13
>
> Reported-by: CAI Qian <caiqian@...hat.com>
> Tested-by: Yang Shukui <yangshukui@...wei.com>
> Signed-off-by: Zhou Chengming <zhouchengming1@...wei.com>
> ---
>  fs/proc/proc_sysctl.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index 5d931bf..c4c90bd 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -718,7 +718,7 @@ static int proc_sys_readdir(struct file *file, struct dir_context *ctx)
>  	ctl_dir = container_of(head, struct ctl_dir, header);
>  
>  	if (!dir_emit_dots(file, ctx))
> -		return 0;
> +		goto out;
>  
>  	pos = 2;
>  
> @@ -728,6 +728,7 @@ static int proc_sys_readdir(struct file *file, struct dir_context *ctx)
>  			break;
>  		}
>  	}
> +out:
>  	sysctl_head_finish(head);
>  	return 0;
>  }