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

From: Vinicius Costa Gomes
Date: Tue Apr 02 2024 - 22:18:37 EST


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