proc,security: move restriction on writing /proc/pid/attr nodes to proc

Processes can only alter their own security attributes via
/proc/pid/attr nodes.  This is presently enforced by each individual
security module and is also imposed by the Linux credentials
implementation, which only allows a task to alter its own credentials.
Move the check enforcing this restriction from the individual
security modules to proc_pid_attr_write() before calling the security hook,
and drop the unnecessary task argument to the security hook since it can
only ever be the current task.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 9 insertions, 4 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 8e7e61b..988c5a7 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2488,6 +2488,12 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 	length = -ESRCH;
 	if (!task)
 		goto out_no_task;
+
+	/* A task may only write its own attributes. */
+	length = -EACCES;
+	if (current != task)
+		goto out;
+
 	if (count > PAGE_SIZE)
 		count = PAGE_SIZE;
 
@@ -2503,14 +2509,13 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 	}
 
 	/* Guard against adverse ptrace interaction */
-	length = mutex_lock_interruptible(&task->signal->cred_guard_mutex);
+	length = mutex_lock_interruptible(&current->signal->cred_guard_mutex);
 	if (length < 0)
 		goto out_free;
 
-	length = security_setprocattr(task,
-				      (char*)file->f_path.dentry->d_name.name,
+	length = security_setprocattr(file->f_path.dentry->d_name.name,
 				      page, count);
-	mutex_unlock(&task->signal->cred_guard_mutex);
+	mutex_unlock(&current->signal->cred_guard_mutex);
 out_free:
 	kfree(page);
 out: