CRED: Differentiate objective and effective subjective credentials on a task

Differentiate the objective and real subjective credentials from the effective
subjective credentials on a task by introducing a second credentials pointer
into the task_struct.

task_struct::real_cred then refers to the objective and apparent real
subjective credentials of a task, as perceived by the other tasks in the
system.

task_struct::cred then refers to the effective subjective credentials of a
task, as used by that task when it's actually running.  These are not visible
to the other tasks in the system.

__task_cred(task) then refers to the objective/real credentials of the task in
question.

current_cred() refers to the effective subjective credentials of the current
task.

prepare_creds() uses the objective creds as a base and commit_creds() changes
both pointers in the task_struct (indeed commit_creds() requires them to be the
same).

override_creds() and revert_creds() change the subjective creds pointer only,
and the former returns the old subjective creds.  These are used by NFSD,
faccessat() and do_coredump(), and will by used by CacheFiles.

In SELinux, current_has_perm() is provided as an alternative to
task_has_perm().  This uses the effective subjective context of current,
whereas task_has_perm() uses the objective/real context of the subject.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 08c3b24ad9a8..2597858035cd 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -149,6 +149,7 @@ extern struct cred init_cred;
         .children       = LIST_HEAD_INIT(tsk.children),                 \
         .sibling        = LIST_HEAD_INIT(tsk.sibling),                  \
         .group_leader   = &tsk,                                         \
+        .real_cred      = &init_cred,                                   \
         .cred           = &init_cred,                                   \
         .cred_exec_mutex =                                              \
                  __MUTEX_INITIALIZER(tsk.cred_exec_mutex),              \