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, 26 insertions, 12 deletions

diff --git a/kernel/cred.c b/kernel/cred.c
index b8bd2f99d8ce..f3ca10660617 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -35,7 +35,7 @@ static struct thread_group_cred init_tgcred = {
  * The initial credentials for the initial task
  */
 struct cred init_cred = {
-        .usage                  = ATOMIC_INIT(3),
+        .usage                  = ATOMIC_INIT(4),
         .securebits             = SECUREBITS_DEFAULT,
         .cap_inheritable        = CAP_INIT_INH_SET,
         .cap_permitted          = CAP_FULL_SET,
@@ -120,6 +120,8 @@ EXPORT_SYMBOL(__put_cred);
  * prepare a new copy, which the caller then modifies and then commits by
  * calling commit_creds().
  *
+ * Preparation involves making a copy of the objective creds for modification.
+ *
  * Returns a pointer to the new creds-to-be if successful, NULL otherwise.
  *
  * Call commit_creds() or abort_creds() to clean up.
@@ -130,7 +132,7 @@ struct cred *prepare_creds(void)
         const struct cred *old;
         struct cred *new;
 
-        BUG_ON(atomic_read(&task->cred->usage) < 1);
+        BUG_ON(atomic_read(&task->real_cred->usage) < 1);
 
         new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
         if (!new)
@@ -262,6 +264,9 @@ error:
  *
  * We share if we can, but under some circumstances we have to generate a new
  * set.
+ *
+ * The new process gets the current process's subjective credentials as its
+ * objective and subjective credentials
  */
 int copy_creds(struct task_struct *p, unsigned long clone_flags)
 {
@@ -278,6 +283,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 #endif
                 clone_flags & CLONE_THREAD
             ) {
+                p->real_cred = get_cred(p->cred);
                 get_cred(p->cred);
                 atomic_inc(&p->cred->user->processes);
                 return 0;
@@ -317,7 +323,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 #endif
 
         atomic_inc(&new->user->processes);
-        p->cred = new;
+        p->cred = p->real_cred = get_cred(new);
         return 0;
 }
 
@@ -326,7 +332,9 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
  * @new: The credentials to be assigned
  *
  * Install a new set of credentials to the current task, using RCU to replace
- * the old set.
+ * the old set.  Both the objective and the subjective credentials pointers are
+ * updated.  This function may not be called if the subjective credentials are
+ * in an overridden state.
  *
  * This function eats the caller's reference to the new credentials.
  *
@@ -338,12 +346,15 @@ int commit_creds(struct cred *new)
         struct task_struct *task = current;
         const struct cred *old;
 
+        BUG_ON(task->cred != task->real_cred);
+        BUG_ON(atomic_read(&task->real_cred->usage) < 2);
         BUG_ON(atomic_read(&new->usage) < 1);
-        BUG_ON(atomic_read(&task->cred->usage) < 1);
 
-        old = task->cred;
+        old = task->real_cred;
         security_commit_creds(new, old);
 
+        get_cred(new); /* we will require a ref for the subj creds too */
+
         /* dumpability changes */
         if (old->euid != new->euid ||
             old->egid != new->egid ||
@@ -369,6 +380,7 @@ int commit_creds(struct cred *new)
          */
         if (new->user != old->user)
                 atomic_inc(&new->user->processes);
+        rcu_assign_pointer(task->real_cred, new);
         rcu_assign_pointer(task->cred, new);
         if (new->user != old->user)
                 atomic_dec(&old->user->processes);
@@ -388,6 +400,8 @@ int commit_creds(struct cred *new)
             new->fsgid != old->fsgid)
                 proc_id_connector(task, PROC_EVENT_GID);
 
+        /* release the old obj and subj refs both */
+        put_cred(old);
         put_cred(old);
         return 0;
 }
@@ -408,11 +422,11 @@ void abort_creds(struct cred *new)
 EXPORT_SYMBOL(abort_creds);
 
 /**
- * override_creds - Temporarily override the current process's credentials
+ * override_creds - Override the current process's subjective credentials
  * @new: The credentials to be assigned
  *
- * Install a set of temporary override credentials on the current process,
- * returning the old set for later reversion.
+ * Install a set of temporary override subjective credentials on the current
+ * process, returning the old set for later reversion.
  */
 const struct cred *override_creds(const struct cred *new)
 {
@@ -424,11 +438,11 @@ const struct cred *override_creds(const struct cred *new)
 EXPORT_SYMBOL(override_creds);
 
 /**
- * revert_creds - Revert a temporary credentials override
+ * revert_creds - Revert a temporary subjective credentials override
  * @old: The credentials to be restored
  *
- * Revert a temporary set of override credentials to an old set, discarding the
- * override set.
+ * Revert a temporary set of override subjective credentials to an old set,
+ * discarding the override set.
  */
 void revert_creds(const struct cred *old)
 {