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, 4 insertions, 1 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 121d655e460d..3443123b0709 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1145,7 +1145,10 @@ struct task_struct {
         struct list_head cpu_timers[3];
 
 /* process credentials */
-        const struct cred *cred;        /* actual/objective task credentials (COW) */
+        const struct cred *real_cred;   /* objective and real subjective task
+                                         * credentials (COW) */
+        const struct cred *cred;        /* effective (overridable) subjective task
+                                         * credentials (COW) */
         struct mutex cred_exec_mutex;   /* execve vs ptrace cred calculation mutex */
 
         char comm[TASK_COMM_LEN]; /* executable name excluding path