Non-controller kernel subsystems may base access restrictions for
cgroup-related syscalls/ioctls on a process' access to the cgroup.
Let's make it easy for other parts of the kernel to check these cgroup
permissions.

Cc: Tejun Heo <t...@kernel.org>
Cc: cgro...@vger.kernel.org
Signed-off-by: Matt Roper <matthew.d.ro...@intel.com>
---
 include/linux/cgroup.h |  1 +
 kernel/cgroup/cgroup.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b1ea2064f247..dd1d1d9813e8 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -100,6 +100,7 @@ struct cgroup_subsys_state 
*css_tryget_online_from_dir(struct dentry *dentry,
 
 struct cgroup *cgroup_get_from_path(const char *path);
 struct cgroup *cgroup_get_from_fd(int fd);
+int cgroup_permission(int fd, int mask);
 
 int cgroup_attach_task_all(struct task_struct *from, struct task_struct *);
 int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from);
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 9e576dc8b566..52d68b226867 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5781,6 +5781,48 @@ struct cgroup *cgroup_get_from_fd(int fd)
 }
 EXPORT_SYMBOL_GPL(cgroup_get_from_fd);
 
+/**
+ * cgroup_permission - check cgroup fd permissions
+ * @fd: fd obtained by open(cgroup)
+ * @mask: Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
+ *
+ * Check for read/write/execute permissions on a cgroup.
+ */
+int cgroup_permission(int fd, int mask)
+{
+       struct file *f;
+       struct inode *inode;
+       struct cgroup_subsys_state *css;
+       int ret;
+
+       f = fget_raw(fd);
+       if (!f)
+               return -EBADF;
+
+       css = css_tryget_online_from_dir(f->f_path.dentry, NULL);
+       if (IS_ERR(css)) {
+               ret = PTR_ERR(css);
+               goto out_file;
+       }
+
+       inode = kernfs_get_inode(f->f_path.dentry->d_sb, css->cgroup->kn);
+       if (!inode) {
+               ret = -ENOMEM;
+               goto out_cgroup;
+       }
+
+       ret = inode_permission(inode, mask);
+       iput(inode);
+
+out_cgroup:
+       cgroup_put(css->cgroup);
+out_file:
+       fput(f);
+
+       return ret;
+}
+EXPORT_SYMBOL_GPL(cgroup_permission);
+
 /*
  * sock->sk_cgrp_data handling.  For more info, see sock_cgroup_data
  * definition in cgroup-defs.h.
-- 
2.14.3

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