[PATCH v1 1/3] cred: Add a light version of override/revert_creds()

[Vinicius Costa Gomes <vinicius.gomes@xxxxxxxxx>]

Add a light version of override/revert_creds(), this should only be
used when the credentials in question will outlive the critical
section and the critical section doesn't change the ->usage of the
credentials.

To make their usage less error prone, introduce cleanup guards to be
used like this:

     cred_guard(credentials_to_override_and_restore);

or this:

     cred_scoped_guard(credentials_to_override_and_restore) {
             /* with credentials overridden */
     }

Suggested-by: Christian Brauner <brauner@xxxxxxxxxx>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@xxxxxxxxx>
---
 include/linux/cred.h | 25 +++++++++++++++++++++++++
 kernel/cred.c        |  6 +++---
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 2976f534a7a3..f4f3d55cd6a2 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -172,6 +172,31 @@ static inline bool cap_ambient_invariant_ok(const struct cred *cred)
 					  cred->cap_inheritable));
 }
 
+/*
+ * Override creds without bumping reference count. Caller must ensure
+ * reference remains valid or has taken reference. Almost always not the
+ * interface you want. Use override_creds()/revert_creds() instead.
+ */
+static inline const struct cred *override_creds_light(const struct cred *override_cred)
+{
+	const struct cred *old = current->cred;
+
+	rcu_assign_pointer(current->cred, override_cred);
+	return old;
+}
+
+static inline void revert_creds_light(const struct cred *revert_cred)
+{
+	rcu_assign_pointer(current->cred, revert_cred);
+}
+
+DEFINE_LOCK_GUARD_1(__cred, struct cred,
+		    _T->lock = (struct cred *)override_creds_light(_T->lock),
+		    revert_creds_light(_T->lock));
+
+#define cred_guard(_cred) guard(__cred)(((struct cred *)_cred))
+#define cred_scoped_guard(_cred) scoped_guard(__cred, ((struct cred *)_cred))
+
 /**
  * get_new_cred_many - Get references on a new set of credentials
  * @cred: The new credentials to reference
diff --git a/kernel/cred.c b/kernel/cred.c
index 075cfa7c896f..da7da250f7c8 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -485,7 +485,7 @@ EXPORT_SYMBOL(abort_creds);
  */
 const struct cred *override_creds(const struct cred *new)
 {
-	const struct cred *old = current->cred;
+	const struct cred *old;
 
 	kdebug("override_creds(%p{%ld})", new,
 	       atomic_long_read(&new->usage));
@@ -499,7 +499,7 @@ const struct cred *override_creds(const struct cred *new)
 	 * visible to other threads under RCU.
 	 */
 	get_new_cred((struct cred *)new);
-	rcu_assign_pointer(current->cred, new);
+	old = override_creds_light(new);
 
 	kdebug("override_creds() = %p{%ld}", old,
 	       atomic_long_read(&old->usage));
@@ -521,7 +521,7 @@ void revert_creds(const struct cred *old)
 	kdebug("revert_creds(%p{%ld})", old,
 	       atomic_long_read(&old->usage));
 
-	rcu_assign_pointer(current->cred, old);
+	revert_creds_light(old);
 	put_cred(override);
 }
 EXPORT_SYMBOL(revert_creds);
-- 
2.44.0