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>
diff --git a/kernel/cred.c b/kernel/cred.c
index b8bd2f9..f3ca106 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -35,7 +35,7 @@
* 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 @@
* 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 @@
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 @@
*
* 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 @@
#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 @@
#endif
atomic_inc(&new->user->processes);
- p->cred = new;
+ p->cred = p->real_cred = get_cred(new);
return 0;
}
@@ -326,7 +332,9 @@
* @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 @@
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 @@
*/
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 @@
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 @@
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 @@
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)
{
