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

Josh Boyer
2015-09-03T13:56:09Z

On Wed, Sep 2, 2015 at 8:14 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> 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>

Just tested this patch without Felipe's previous version on all my
machines. The splat is indeed gone.

Tested-by: Josh Boyer <jwboyer@fedoraproject.org>

josh

>
> 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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