Re: [PATCH] cgroup: fix invalid rcu dereference

[Aristeu Rozanski <aris@redhat.com>]

On Fri, Sep 14, 2012 at 11:40:12AM +0200, Jiri Slaby wrote:
> 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.

ugh, missed that

> 
> 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
> -------------------------------
> /home/latest/linux/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
> 
> Signed-off-by: Jiri Slaby <jslaby@suse.cz>
> Cc: aris@redhat.com
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: lizefan@huawei.com
> ---
> 
> And this should fix it.
> 
>  security/device_cgroup.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> index 8d21ded..2178886 100644
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -524,10 +524,10 @@ 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;
>  
> @@ -538,6 +538,7 @@ static int __devcgroup_check_permission(struct dev_cgroup *dev_cgroup,
>  	ex.access = access;
>  
>  	rcu_read_lock();
> +	dev_cgroup = task_devcgroup(current);
>  	rc = may_access(dev_cgroup, &ex);
>  	rcu_read_unlock();
>  
> @@ -549,7 +550,6 @@ 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);
>  	short type, access = 0;
>  
>  	if (S_ISBLK(inode->i_mode))
> @@ -561,13 +561,12 @@ 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),
> -					    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))
> @@ -578,7 +577,7 @@ int devcgroup_inode_mknod(int mode, dev_t dev)
>  	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);
>  
>  }

thanks Jiri
Acked-by: Aristeu Rozanski <aris@redhat.com>

-- 
Aristeu