Re: include/linux/cgroup.h:566 suspicious rcu_dereference_check()usage!

From: Paul E. McKenney
Date: Tue Oct 09 2012 - 23:25:01 EST

On Tue, Oct 09, 2012 at 06:08:59PM -0700, Sergey Senozhatsky wrote:
> On (10/08/12 12:49), Paul E. McKenney wrote:
> >
> > ------------------------------------------------------------------------
> >
> > device_cgroup: Restore rcu_read_lock() protection to devcgroup_inode_mknod()
> >
> > Commit ad676077 (device_cgroup: convert device_cgroup internally to
> > policy + exceptions) restructured devcgroup_inode_mknod(), removing
> > rcu_read_lock() in the process. However, RCU read-side protection
> > is required by the call to task_devcgroup(), so this commit restores
> > the rcu_read_lock() and rcu_read_unlock().
> >
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> >
> > diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> > index 44dfc41..c686110 100644
> > --- a/security/device_cgroup.c
> > +++ b/security/device_cgroup.c
> > @@ -576,9 +576,12 @@ int __devcgroup_inode_permission(struct inode *inode, int mask)
> >
> > int devcgroup_inode_mknod(int mode, dev_t dev)
> > {
> > - struct dev_cgroup *dev_cgroup = task_devcgroup(current);
> > + struct dev_cgroup *dev_cgroup;
> > + int ret;
> > short type;
> >
> > + rcu_read_lock();
> > + dev_cgroup = task_devcgroup(current);
> > if (!S_ISBLK(mode) && !S_ISCHR(mode))
> > return 0;
> >
> > @@ -587,7 +590,9 @@ int devcgroup_inode_mknod(int mode, dev_t dev)
> > else
> > type = DEV_CHAR;
> >
> > - return __devcgroup_check_permission(dev_cgroup, type, MAJOR(dev),
> > + ret = __devcgroup_check_permission(dev_cgroup, type, MAJOR(dev),
> > MINOR(dev), ACC_MKNOD);
> > + rcu_read_unlock();
> > + return ret;
> >
> > }
> >
>
>
> I believe the same should be done for __devcgroup_inode_permission() as well. And we
> probably can call task_devcgroup() and rcu_read_lock() after "S_ISBLK(mode) && !S_ISCHR(mode)"
> checks (I guess we also need to unlock RCU on `return 0').

Looks sane to me! Dropping my patch.

Thanx, Paul

> ----------------------------------------------------------------------------
>
> Commit ad676077
> | Author: Aristeu Rozanski <aris@xxxxxxxxxx>
> | Date: Thu Oct 4 17:15:17 2012 -0700
> | device_cgroup: convert device_cgroup internally to policy + exceptions
>
> moved RCU read-side protection from devcgroup_inode_mknod(), which, however is required
> by task_devcgroup(). Patch also add RCU read-side protection to __devcgroup_inode_permission()
> function, introduced in commit ad676077.
>
> [ 0.946303] include/linux/cgroup.h:566 suspicious rcu_dereference_check() usage!
> [ 0.946511]
> [ 0.946606] 2 locks held by kdevtmpfs/28:
> [ 0.946684] #0: (sb_writers){.+.+.+}, at: [<ffffffff81144bcb>] mnt_want_write+0x24/0x4b
> [ 0.947083] #1: (&sb->s_type->i_mutex_key#3/1){+.+.+.}, at: [<ffffffff81133d04>] kern_path_create+0x83/0x144
> [ 0.947598]
> [ 0.947787] Call Trace:
> [ 0.947868] [<ffffffff81089644>] lockdep_rcu_suspicious+0x109/0x112
> [ 0.947958] [<ffffffff81258fa0>] devcgroup_inode_mknod+0x9e/0xee
> [ 0.948043] [<ffffffff81132ee7>] vfs_mknod+0x8a/0xed
> [ 0.948129] [<ffffffff813b98af>] handle_create.isra.2+0x144/0x1b5
> [ 0.948214] [<ffffffff813b99bf>] ? devtmpfsd+0x9f/0x138
> [ 0.948298] [<ffffffff81295d5c>] ? do_raw_spin_lock+0x67/0xde
> [ 0.948384] [<ffffffff81295e92>] ? do_raw_spin_unlock+0x8f/0x98
> [ 0.948469] [<ffffffff813b9920>] ? handle_create.isra.2+0x1b5/0x1b5
> [ 0.948554] [<ffffffff813b9a04>] devtmpfsd+0xe4/0x138
> [ 0.948638] [<ffffffff813b9920>] ? handle_create.isra.2+0x1b5/0x1b5
> [ 0.948724] [<ffffffff810582b6>] kthread+0xd5/0xdd
> [ 0.948814] [<ffffffff814db664>] kernel_thread_helper+0x4/0x10
> [ 0.948900] [<ffffffff814d2973>] ? retint_restore_args+0x13/0x13
> [ 0.948985] [<ffffffff810581e1>] ? __init_kthread_worker+0x5a/0x5a
> [ 0.949069] [<ffffffff814db660>] ? gs_change+0x13/0x13
>
>
> devcgroup_inode_mknod() part submitted by Paul E. McKenney.
>
>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
>
> ---
>
> security/device_cgroup.c | 21 +++++++++++++++++----
> 1 file changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> index 44dfc41..043eb00 100644
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -558,7 +558,8 @@ static int __devcgroup_check_permission(struct dev_cgroup *dev_cgroup,
>
> int __devcgroup_inode_permission(struct inode *inode, int mask)
> {
> - struct dev_cgroup *dev_cgroup = task_devcgroup(current);
> + struct dev_cgroup *dev_cgroup;
> + int ret;
> short type, access = 0;
>
> if (S_ISBLK(inode->i_mode))
> @@ -570,13 +571,20 @@ int __devcgroup_inode_permission(struct inode *inode, int mask)
> if (mask & MAY_READ)
> access |= ACC_READ;
>
> - return __devcgroup_check_permission(dev_cgroup, type, imajor(inode),
> + rcu_read_lock();
> +
> + dev_cgroup = task_devcgroup(current);
> + ret = __devcgroup_check_permission(dev_cgroup, type, imajor(inode),
> iminor(inode), access);
> +
> + rcu_read_unlock();
> + return ret;
> }
>
> int devcgroup_inode_mknod(int mode, dev_t dev)
> {
> - struct dev_cgroup *dev_cgroup = task_devcgroup(current);
> + struct dev_cgroup *dev_cgroup;
> + int ret;
> short type;
>
> if (!S_ISBLK(mode) && !S_ISCHR(mode))
> @@ -587,7 +595,12 @@ int devcgroup_inode_mknod(int mode, dev_t dev)
> else
> type = DEV_CHAR;
>
> - return __devcgroup_check_permission(dev_cgroup, type, MAJOR(dev),
> + rcu_read_lock();
> +
> + dev_cgroup = task_devcgroup(current);
> + ret = __devcgroup_check_permission(dev_cgroup, type, MAJOR(dev),
> MINOR(dev), ACC_MKNOD);
>
> + rcu_read_unlock();
> + return ret;
> }
>
> --
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