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, 2 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 82a7948a664e..af0d0f04585c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -146,6 +146,7 @@ void __put_task_struct(struct task_struct *tsk)
         WARN_ON(atomic_read(&tsk->usage));
         WARN_ON(tsk == current);
 
+        put_cred(tsk->real_cred);
         put_cred(tsk->cred);
         delayacct_tsk_free(tsk);
 
@@ -961,10 +962,10 @@ static struct task_struct *copy_process(unsigned long clone_flags,
         DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
 #endif
         retval = -EAGAIN;
-        if (atomic_read(&p->cred->user->processes) >=
+        if (atomic_read(&p->real_cred->user->processes) >=
                         p->signal->rlim[RLIMIT_NPROC].rlim_cur) {
                 if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE) &&
-                    p->cred->user != current->nsproxy->user_ns->root_user)
+                    p->real_cred->user != current->nsproxy->user_ns->root_user)
                         goto bad_fork_free;
         }
 
@@ -1278,6 +1279,7 @@ bad_fork_cleanup_put_domain:
         module_put(task_thread_info(p)->exec_domain->module);
 bad_fork_cleanup_count:
         atomic_dec(&p->cred->user->processes);
+        put_cred(p->real_cred);
         put_cred(p->cred);
 bad_fork_free:
         free_task(p);