Re: [PATCH v2] security: device_cgroup: fix RCU lockdep splat

[Paul E. McKenney]

On Wed, Sep 02, 2015 at 12:24:50PM -0400, Tejun Heo wrote:
> 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?

Gah!  Please see below.

							Thanx, Paul

------------------------------------------------------------------------

security/device_cgroup: Fix RCU_LOCKDEP_WARN() condition

f78f5b90c4ff ("rcu: Rename rcu_lockdep_assert() to RCU_LOCKDEP_WARN()")
introduced a bug by incorrectly inverting the condition when moving from
rcu_lockdep_assert() to RCU_LOCKDEP_WARN().  This commit therefore fixes
the inversion.

Reported-by: Felipe Balbi <balbi@xxxxxx>
Reported-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Serge Hallyn <serge@xxxxxxxxxx>

diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 73455089feef..03c1652c9a1f 100644
--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -401,7 +401,7 @@ static bool verify_new_ex(struct dev_cgroup *dev_cgroup,
 	bool match = false;
 
 	RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&
-			 lockdep_is_held(&devcgroup_mutex),
+			 !lockdep_is_held(&devcgroup_mutex),
 			 "device_cgroup:verify_new_ex called without proper synchronization");
 
 	if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW) {

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