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

2015-09-03 Thread Josh Boyer 
On Wed, Sep 2, 2015 at 8:14 PM, 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] [] (verify_new_ex) from [] 
>> > (devcgroup_access_write+0x374/0x658)
>> > [   12.137426] [] (devcgroup_access_write) from [] 
>> > (cgroup_file_write+0x28/0x1bc)
>> > [   12.146796] [] (cgroup_file_write) from [] 
>> > (kernfs_fop_write+0xc0/0x1b8)
>> > [   12.155620] [] (kernfs_fop_write) from [] 
>> > (__vfs_write+0x1c/0xd8)
>> > [   12.163783] [] (__vfs_write) from [] 
>> > (vfs_write+0x90/0x16c)
>> > [   12.171426] [] (vfs_write) from [] 
>> > (SyS_write+0x44/0x9c)
>> > [   12.178806] [] (SyS_write) from [] 
>> > (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 
> Reported-by: Tejun Heo 
> Signed-off-by: Paul E. McKenney 
> Cc: Serge Hallyn 

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

Tested-by: Josh Boyer 

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(_mutex),
> +!lockdep_is_held(_mutex),
>  "device_cgroup:verify_new_ex called without proper 
> synchronization");
>
> if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW) {
>
> --
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Re: [PATCH v2] security: device_cgroup: fix RCU lockdep splat

2015-09-03 Thread Serge E. Hallyn 
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] [] (verify_new_ex) from [] 
> > > (devcgroup_access_write+0x374/0x658)
> > > [   12.137426] [] (devcgroup_access_write) from [] 
> > > (cgroup_file_write+0x28/0x1bc)
> > > [   12.146796] [] (cgroup_file_write) from [] 
> > > (kernfs_fop_write+0xc0/0x1b8)
> > > [   12.155620] [] (kernfs_fop_write) from [] 
> > > (__vfs_write+0x1c/0xd8)
> > > [   12.163783] [] (__vfs_write) from [] 
> > > (vfs_write+0x90/0x16c)
> > > [   12.171426] [] (vfs_write) from [] 
> > > (SyS_write+0x44/0x9c)
> > > [   12.178806] [] (SyS_write) from [] 
> > > (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 
> Reported-by: Tejun Heo 
> Signed-off-by: Paul E. McKenney 
> Cc: Serge Hallyn 

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

Acked-by: Serge Hallyn 

> 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(_mutex),
> +  !lockdep_is_held(_mutex),
>"device_cgroup:verify_new_ex called without proper 
> synchronization");
>  
>   if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW) {
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Re: [PATCH v2] security: device_cgroup: fix RCU lockdep splat

2015-09-03 Thread Paul E. McKenney 
On Thu, Sep 03, 2015 at 08:48:39AM -0500, Serge E. Hallyn wrote:
> 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] [] (verify_new_ex) from [] 
> > > > (devcgroup_access_write+0x374/0x658)
> > > > [   12.137426] [] (devcgroup_access_write) from [] 
> > > > (cgroup_file_write+0x28/0x1bc)
> > > > [   12.146796] [] (cgroup_file_write) from [] 
> > > > (kernfs_fop_write+0xc0/0x1b8)
> > > > [   12.155620] [] (kernfs_fop_write) from [] 
> > > > (__vfs_write+0x1c/0xd8)
> > > > [   12.163783] [] (__vfs_write) from [] 
> > > > (vfs_write+0x90/0x16c)
> > > > [   12.171426] [] (vfs_write) from [] 
> > > > (SyS_write+0x44/0x9c)
> > > > [   12.178806] [] (SyS_write) from [] 
> > > > (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 
> > Reported-by: Tejun Heo 
> > Signed-off-by: Paul E. McKenney 
> > Cc: Serge Hallyn 
> 
> Oh, makes sense :)  (didn't see the original patch when it came by, sorry)

I should have CCed you, apologies for failing to do so.

> Acked-by: Serge Hallyn 

Added, thank you!

Thanx, Paul

> > 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(_mutex),
> > +!lockdep_is_held(_mutex),
> >  "device_cgroup:verify_new_ex called without proper 
> > synchronization");
> >  
> > if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW) {
> 

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

2015-09-03 Thread Paul E. McKenney 
On Thu, Sep 03, 2015 at 09:56:05AM -0400, Josh Boyer wrote:
> On Wed, Sep 2, 2015 at 8:14 PM, 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] [] (verify_new_ex) from [] 
> >> > (devcgroup_access_write+0x374/0x658)
> >> > [   12.137426] [] (devcgroup_access_write) from [] 
> >> > (cgroup_file_write+0x28/0x1bc)
> >> > [   12.146796] [] (cgroup_file_write) from [] 
> >> > (kernfs_fop_write+0xc0/0x1b8)
> >> > [   12.155620] [] (kernfs_fop_write) from [] 
> >> > (__vfs_write+0x1c/0xd8)
> >> > [   12.163783] [] (__vfs_write) from [] 
> >> > (vfs_write+0x90/0x16c)
> >> > [   12.171426] [] (vfs_write) from [] 
> >> > (SyS_write+0x44/0x9c)
> >> > [   12.178806] [] (SyS_write) from [] 
> >> > (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 
> > Reported-by: Tejun Heo 
> > Signed-off-by: Paul E. McKenney 
> > Cc: Serge Hallyn 
> 
> Just tested this patch without Felipe's previous version on all my
> machines.  The splat is indeed gone.
> 
> Tested-by: Josh Boyer 

Thank you, added!

Thanx, Paul

> 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(_mutex),
> > +!lockdep_is_held(_mutex),
> >  "device_cgroup:verify_new_ex called without proper 
> > synchronization");
> >
> > if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW) {
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> > the body of a message to majord...@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/
> 

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

2015-09-02 Thread Tejun Heo 
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] [] (verify_new_ex) from [] 
> (devcgroup_access_write+0x374/0x658)
> [   12.137426] [] (devcgroup_access_write) from [] 
> (cgroup_file_write+0x28/0x1bc)
> [   12.146796] [] (cgroup_file_write) from [] 
> (kernfs_fop_write+0xc0/0x1b8)
> [   12.155620] [] (kernfs_fop_write) from [] 
> (__vfs_write+0x1c/0xd8)
> [   12.163783] [] (__vfs_write) from [] 
> (vfs_write+0x90/0x16c)
> [   12.171426] [] (vfs_write) from [] 
> (SyS_write+0x44/0x9c)
> [   12.178806] [] (SyS_write) from [] 
> (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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Re: [PATCH v2] security: device_cgroup: fix RCU lockdep splat

2015-09-02 Thread Josh Boyer 
On Wed, Sep 2, 2015 at 9:12 AM, 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!
> [   12.033576] other info that might help us debug this:
>
> [   12.041942] rcu_scheduler_active = 1, debug_locks = 0
> [   12.048796] 4 locks held by systemd/1:
> [   12.052700]  #0:  (sb_writers#7){.+.+.+}, at: [] 
> __sb_start_write+0x8c/0xb0
> [   12.060954]  #1:  (>mutex){+.+.+.}, at: [] 
> kernfs_fop_write+0x50/0x1b8
> [   12.069085]  #2:  (s_active#30){.+}, at: [] 
> kernfs_fop_write+0x58/0x1b8
> [   12.077310]  #3:  (devcgroup_mutex){+.+...}, at: [] 
> devcgroup_access_write+0x20/0x658
> [   12.086575] stack backtrace:
> [   12.091124] CPU: 0 PID: 1 Comm: systemd Not tainted 4.2.0-next-20150831 
> #1154
> [   12.098609] Hardware name: Generic AM43 (Flattened Device Tree)
> [   12.104807] [] (unwind_backtrace) from [] 
> (show_stack+0x10/0x14)
> [   12.112924] [] (show_stack) from [] 
> (dump_stack+0x84/0x9c)
> [   12.120491] [] (dump_stack) from [] 
> (verify_new_ex+0xc4/0xdc)
> [   12.128326] [] (verify_new_ex) from [] 
> (devcgroup_access_write+0x374/0x658)
> [   12.137426] [] (devcgroup_access_write) from [] 
> (cgroup_file_write+0x28/0x1bc)
> [   12.146796] [] (cgroup_file_write) from [] 
> (kernfs_fop_write+0xc0/0x1b8)
> [   12.155620] [] (kernfs_fop_write) from [] 
> (__vfs_write+0x1c/0xd8)
> [   12.163783] [] (__vfs_write) from [] 
> (vfs_write+0x90/0x16c)
> [   12.171426] [] (vfs_write) from [] 
> (SyS_write+0x44/0x9c)
> [   12.178806] [] (SyS_write) from [] 
> (ret_fast_syscall+0x0/0x1c)
>
> Fix it by making sure rcu_read_lock() is held
> around calls to parent_has_perm().
>
> Signed-off-by: Felipe Balbi 

This cleared up the splat on all my machines and I don't see any other
side effects (even with lockdep enabled).  Thanks!

Tested-by: Josh Boyer 

josh

> ---
>
> Changes since v1:
> - move rcu_read_lock/unlock to wrap parent_has_perm()
>
>  security/device_cgroup.c | 7 ++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> index 73455089feef..dd77ed206fa4 100644
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -608,6 +608,7 @@ static int devcgroup_update_access(struct dev_cgroup 
> *devcgroup,
> int count, rc = 0;
> struct dev_exception_item ex;
> struct dev_cgroup *parent = css_to_devcgroup(devcgroup->css.parent);
> +   int ret;
>
> if (!capable(CAP_SYS_ADMIN))
> return -EPERM;
> @@ -734,7 +735,11 @@ static int devcgroup_update_access(struct dev_cgroup 
> *devcgroup,
> break;
> }
>
> -   if (!parent_has_perm(devcgroup, ))
> +   rcu_read_lock();
> +   ret = parent_has_perm(devcgroup, );
> +   rcu_read_unlock();
> +
> +   if (!ret)
> return -EPERM;
> rc = dev_exception_add(devcgroup, );
> break;
> --
> 2.5.0
>
> --
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Re: [PATCH v2] security: device_cgroup: fix RCU lockdep splat

2015-09-02 Thread 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] [] (verify_new_ex) from [] 
> > (devcgroup_access_write+0x374/0x658)
> > [   12.137426] [] (devcgroup_access_write) from [] 
> > (cgroup_file_write+0x28/0x1bc)
> > [   12.146796] [] (cgroup_file_write) from [] 
> > (kernfs_fop_write+0xc0/0x1b8)
> > [   12.155620] [] (kernfs_fop_write) from [] 
> > (__vfs_write+0x1c/0xd8)
> > [   12.163783] [] (__vfs_write) from [] 
> > (vfs_write+0x90/0x16c)
> > [   12.171426] [] (vfs_write) from [] 
> > (SyS_write+0x44/0x9c)
> > [   12.178806] [] (SyS_write) from [] 
> > (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 
Reported-by: Tejun Heo 
Signed-off-by: Paul E. McKenney 
Cc: Serge Hallyn 

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(_mutex),
+!lockdep_is_held(_mutex),
 "device_cgroup:verify_new_ex called without proper 
synchronization");
 
if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW) {

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

2015-09-02 Thread Felipe Balbi 
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!
[   12.033576] other info that might help us debug this:

[   12.041942] rcu_scheduler_active = 1, debug_locks = 0
[   12.048796] 4 locks held by systemd/1:
[   12.052700]  #0:  (sb_writers#7){.+.+.+}, at: [] 
__sb_start_write+0x8c/0xb0
[   12.060954]  #1:  (>mutex){+.+.+.}, at: [] 
kernfs_fop_write+0x50/0x1b8
[   12.069085]  #2:  (s_active#30){.+}, at: [] 
kernfs_fop_write+0x58/0x1b8
[   12.077310]  #3:  (devcgroup_mutex){+.+...}, at: [] 
devcgroup_access_write+0x20/0x658
[   12.086575] stack backtrace:
[   12.091124] CPU: 0 PID: 1 Comm: systemd Not tainted 4.2.0-next-20150831 #1154
[   12.098609] Hardware name: Generic AM43 (Flattened Device Tree)
[   12.104807] [] (unwind_backtrace) from [] 
(show_stack+0x10/0x14)
[   12.112924] [] (show_stack) from [] 
(dump_stack+0x84/0x9c)
[   12.120491] [] (dump_stack) from [] 
(verify_new_ex+0xc4/0xdc)
[   12.128326] [] (verify_new_ex) from [] 
(devcgroup_access_write+0x374/0x658)
[   12.137426] [] (devcgroup_access_write) from [] 
(cgroup_file_write+0x28/0x1bc)
[   12.146796] [] (cgroup_file_write) from [] 
(kernfs_fop_write+0xc0/0x1b8)
[   12.155620] [] (kernfs_fop_write) from [] 
(__vfs_write+0x1c/0xd8)
[   12.163783] [] (__vfs_write) from [] 
(vfs_write+0x90/0x16c)
[   12.171426] [] (vfs_write) from [] (SyS_write+0x44/0x9c)
[   12.178806] [] (SyS_write) from [] 
(ret_fast_syscall+0x0/0x1c)

Fix it by making sure rcu_read_lock() is held
around calls to parent_has_perm().

Signed-off-by: Felipe Balbi 
---

Changes since v1:
- move rcu_read_lock/unlock to wrap parent_has_perm()

 security/device_cgroup.c | 7 ++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 73455089feef..dd77ed206fa4 100644
--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -608,6 +608,7 @@ static int devcgroup_update_access(struct dev_cgroup 
*devcgroup,
int count, rc = 0;
struct dev_exception_item ex;
struct dev_cgroup *parent = css_to_devcgroup(devcgroup->css.parent);
+   int ret;
 
if (!capable(CAP_SYS_ADMIN))
return -EPERM;
@@ -734,7 +735,11 @@ static int devcgroup_update_access(struct dev_cgroup 
*devcgroup,
break;
}
 
-   if (!parent_has_perm(devcgroup, ))
+   rcu_read_lock();
+   ret = parent_has_perm(devcgroup, );
+   rcu_read_unlock();
+
+   if (!ret)
return -EPERM;
rc = dev_exception_add(devcgroup, );
break;
-- 
2.5.0

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