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SubjectRe: [PATCH v2] security: device_cgroup: fix RCU lockdep splat
On Wed, Sep 02, 2015 at 05:14:33PM -0700, Paul E. McKenney wrote:
> 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@ti.com>
> Reported-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Serge Hallyn <serge@hallyn.com>

Oh, makes sense :) (didn't see the original patch when it came by, sorry)

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

> 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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