linux-kernel - Re: [PATCH v2] security: device

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Message-ID: <20150902162450.GJ22326@mtj.duckdns.org>
Date:	Wed, 2 Sep 2015 12:24:50 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Felipe Balbi <balbi@...com>
Cc:	James Morris <james.l.morris@...cle.com>, serge@...lyn.com,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] security: device_cgroup: fix RCU lockdep splat

cc'ing Paul.

On Wed, Sep 02, 2015 at 08:12:28AM -0500, Felipe Balbi wrote:
> while booting AM437x device, the following splat
> triggered:
> 
> [   12.005238] ===============================
> [   12.009749] [ INFO: suspicious RCU usage. ]
> [   12.014116] 4.2.0-next-20150831 #1154 Not tainted
> [   12.019050] -------------------------------
> [   12.023408] security/device_cgroup.c:405 device_cgroup:verify_new_ex called without proper synchronization!
...
> [   12.128326] [<c0317a04>] (verify_new_ex) from [<c0317f50>] (devcgroup_access_write+0x374/0x658)
> [   12.137426] [<c0317f50>] (devcgroup_access_write) from [<c00d2800>] (cgroup_file_write+0x28/0x1bc)
> [   12.146796] [<c00d2800>] (cgroup_file_write) from [<c01f1670>] (kernfs_fop_write+0xc0/0x1b8)
> [   12.155620] [<c01f1670>] (kernfs_fop_write) from [<c0177c94>] (__vfs_write+0x1c/0xd8)
> [   12.163783] [<c0177c94>] (__vfs_write) from [<c0178594>] (vfs_write+0x90/0x16c)
> [   12.171426] [<c0178594>] (vfs_write) from [<c0178db4>] (SyS_write+0x44/0x9c)
> [   12.178806] [<c0178db4>] (SyS_write) from [<c000f680>] (ret_fast_syscall+0x0/0x1c)

This shouldn't be happening because devcgroup_access_write() always
grabs devcgroup_mutex.  Looking at the log, the culprit seems to be
f78f5b90c4ff ("rcu: Rename rcu_lockdep_assert() to
RCU_LOCKDEP_WARN()").  It missed the bang for the second test while
inverting it, so adding rcu_read_lock() isn't the right fix here.

Paul, can you please fix it?

Thanks.

-- 
tejun
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