Quoting Aristeu Rozanski (aris@xxxxxxxxxx):
> From: Jiri Slaby <jslaby@xxxxxxx>
>
> Commit "device_cgroup: convert device_cgroup internally to policy +
> exceptions" removed rcu locks which are needed in task_devcgroup
> called in this chain: devcgroup_inode_mknod OR
> __devcgroup_inode_permission -> __devcgroup_inode_permission ->
> task_devcgroup -> task_subsys_state -> task_subsys_state_check.
>
> Change the code so that task_devcgroup is safely called with rcu read
> lock held.
>
> ===============================
> [ INFO: suspicious RCU usage. ]
> 3.6.0-rc5-next-20120913+ #42 Not tainted
> -------------------------------
> include/linux/cgroup.h:553 suspicious rcu_dereference_check() usage!
>
> other info that might help us debug this:
>
> rcu_scheduler_active = 1, debug_locks = 0
> 2 locks held by kdevtmpfs/23:
> #0: (sb_writers){.+.+.+}, at: [<ffffffff8116873f>]
> mnt_want_write+0x1f/0x50
> #1: (&sb->s_type->i_mutex_key#3/1){+.+.+.}, at: [<ffffffff811558af>]
> kern_path_create+0x7f/0x170
>
> stack backtrace:
> Pid: 23, comm: kdevtmpfs Not tainted 3.6.0-rc5-next-20120913+ #42
> Call Trace:
> [<ffffffff810c638d>] lockdep_rcu_suspicious+0xfd/0x130
> [<ffffffff8121541d>] devcgroup_inode_mknod+0x19d/0x240
> [<ffffffff8107bf54>] ? ns_capable+0x44/0x80
> [<ffffffff81156b21>] vfs_mknod+0x71/0xf0
> [<ffffffff813a8332>] handle_create.isra.2+0x72/0x200
> [<ffffffff813a85d4>] devtmpfsd+0x114/0x140
> [<ffffffff813a84c0>] ? handle_create.isra.2+0x200/0x200
> [<ffffffff81093ad6>] kthread+0xd6/0xe0
> [<ffffffff81654f24>] kernel_thread_helper+0x4/0x10
> [<ffffffff8165369d>] ? retint_restore_args+0xe/0xe
> [<ffffffff81093a00>] ? kthread_create_on_node+0x140/0x140
> [<ffffffff81654f20>] ? gs_change+0xb/0xb
>
> Cc: Dave Jones <davej@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Li Zefan <lizefan@xxxxxxxxxx>
> Cc: James Morris <jmorris@xxxxxxxxx>
> Cc: Pavel Emelyanov <xemul@xxxxxxxxxx>
> Cc: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
Acked-by: Serge E. Hallyn <serge.hallyn@xxxxxxxxxx>
> Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
>
> ---
>
> And this should fix it.
>
> security/device_cgroup.c | 15 +++++++--------
> 1 file changed, 7 insertions(+), 8 deletions(-)
>
> --- github.orig/security/device_cgroup.c 2012-10-17 11:11:08.514793906 -0400
> +++ github/security/device_cgroup.c 2012-10-19 16:35:37.936804289 -0400
> @@ -533,10 +533,10 @@ struct cgroup_subsys devices_subsys = {
> *
> * returns 0 on success, -EPERM case the operation is not permitted
> */
> -static int __devcgroup_check_permission(struct dev_cgroup *dev_cgroup,
> - short type, u32 major, u32 minor,
> +static int __devcgroup_check_permission(short type, u32 major, u32 minor,
> short access)
> {
> + struct dev_cgroup *dev_cgroup;
> struct dev_exception_item ex;
> int rc;
>
> @@ -547,6 +547,7 @@ memset(&ex, 0, sizeof(ex));
> ex.access = access;
>
> rcu_read_lock();
> + dev_cgroup = task_devcgroup(current);
> rc = may_access(dev_cgroup, &ex);
> rcu_read_unlock();
>
> @@ -558,7 +559,6 @@ return 0;
>
> int __devcgroup_inode_permission(struct inode *inode, int mask)
> {
> - struct dev_cgroup *dev_cgroup = task_devcgroup(current);
> short type, access = 0;
>
> if (S_ISBLK(inode->i_mode))
> @@ -570,13 +570,12 @@ short type, access = 0;
> if (mask & MAY_READ)
> access |= ACC_READ;
>
> - return __devcgroup_check_permission(dev_cgroup, type, imajor(inode),
> - iminor(inode), access);
> + return __devcgroup_check_permission(type, imajor(inode), iminor(inode),
> + access);
> }
>
> int devcgroup_inode_mknod(int mode, dev_t dev)
> {
> - struct dev_cgroup *dev_cgroup = task_devcgroup(current);
> short type;
>
> if (!S_ISBLK(mode) && !S_ISCHR(mode))
> @@ -587,7 +586,7 @@ return 0;
> else
> type = DEV_CHAR;
>
> - return __devcgroup_check_permission(dev_cgroup, type, MAJOR(dev),
> - MINOR(dev), ACC_MKNOD);
> + return __devcgroup_check_permission(type, MAJOR(dev), MINOR(dev),
> + ACC_MKNOD);
>
> }
>
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