[PATCH v2 06/15] lsm: Add the bpf_lsm_policy_acquire kfunc

[Justin Suess <utilityemal77@gmail.com>]

Add the kfunc acquiring a reference on a policy object the program
does not own:

  bpf_lsm_policy_acquire(object)        KF_ACQUIRE|KF_RCU|KF_RET_NULL

bpf_kptr_xchg() is the only way to take an owned pointer out of a map
kptr field, and it empties the slot: concurrent executions of an
enforcement program would race for the one stored reference.  Modeled
after bpf_task_acquire(), this kfunc removes the exclusivity: a
program loads the kptr field with a plain read under
bpf_rcu_read_lock(), acquires its own reference through the
policy_object_get hook, and leaves the map slot untouched.  The
acquired reference survives bpf_rcu_read_unlock(), carrying over to a
sleepable bpf_lsm_policy_apply_bprm() call, and is released with
bpf_lsm_policy_release().

Adding struct lsm_policy_object to the verifier's rcu_protected_types
set makes the plain load yield an RCU-protected pointer instead of an
untrusted one.  This is where the policy object contract's RCU
requirements become load-bearing: the kfunc and the get hook examine
the object concurrently with a possible last put, which is safe
because implementations free only after an RCU grace period and
acquire with inc-not-zero semantics.  A failed get makes the kfunc
return NULL, per KF_RET_NULL.

The kfunc does not sleep and is meaningful wherever a policy object
pointer can be loaded, so the filter adds no per-kfunc rule.

Cc: Paul Moore <paul@paul-moore.com>
Cc: KP Singh <kpsingh@kernel.org>
Signed-off-by: Justin Suess <utilityemal77@gmail.com>
---
 kernel/bpf/verifier.c     |  3 +++
 security/bpf_lsm_kfuncs.c | 34 +++++++++++++++++++++++++++++++++-
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7aa47342dc65..ba9972c572e1 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4394,6 +4394,9 @@ BTF_ID(struct, task_struct)
 #ifdef CONFIG_CRYPTO
 BTF_ID(struct, bpf_crypto_ctx)
 #endif
+#ifdef CONFIG_BPF_LSM
+BTF_ID(struct, lsm_policy_object)
+#endif
 BTF_SET_END(rcu_protected_types)
 
 static bool rcu_protected_object(const struct btf *btf, u32 btf_id)
diff --git a/security/bpf_lsm_kfuncs.c b/security/bpf_lsm_kfuncs.c
index 988dcd6f4dd9..43a4bf57fd31 100644
--- a/security/bpf_lsm_kfuncs.c
+++ b/security/bpf_lsm_kfuncs.c
@@ -14,6 +14,36 @@
 
 __bpf_kfunc_start_defs();
 
+/**
+ * bpf_lsm_policy_acquire - Acquire a reference on a shared policy object
+ * @object: RCU-protected pointer to a policy object, e.g. loaded from
+ *          a map kptr field under bpf_rcu_read_lock()
+ *
+ * Acquire a reference of its own on a policy object the program does
+ * not own, so that any number of concurrent program executions can
+ * use the object shared through one map kptr field, without emptying
+ * it as bpf_kptr_xchg() would.  The returned reference stays valid
+ * after bpf_rcu_read_unlock() and must be released with
+ * bpf_lsm_policy_release().
+ *
+ * Return: A referenced policy object, or NULL if the object's
+ * reference count concurrently dropped to zero.
+ */
+__bpf_kfunc struct lsm_policy_object *
+bpf_lsm_policy_acquire(struct lsm_policy_object *object)
+{
+	struct lsm_static_call *scall;
+
+	lsm_for_each_hook(scall, policy_object_get) {
+		if (scall->hl->lsmid->id != object->lsmid)
+			continue;
+		if (scall->hl->hook.policy_object_get(object))
+			return NULL;
+		return object;
+	}
+	return NULL;
+}
+
 /**
  * bpf_lsm_policy_from_fd - Get an LSM policy object from a fd
  * @fd: file descriptor referring to a policy object, resolved in the
@@ -57,7 +87,8 @@ __bpf_kfunc struct lsm_policy_object *bpf_lsm_policy_from_fd(int fd, u32 flags)
  * bpf_lsm_policy_release - Release a policy object reference
  * @object: policy object to release
  *
- * Release a reference acquired with bpf_lsm_policy_from_fd().
+ * Release a reference acquired with bpf_lsm_policy_from_fd() or
+ * bpf_lsm_policy_acquire().
  */
 __bpf_kfunc void bpf_lsm_policy_release(struct lsm_policy_object *object)
 {
@@ -83,6 +114,7 @@ CFI_NOSEAL(bpf_lsm_policy_release_dtor);
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(bpf_lsm_policy_kfunc_ids)
+BTF_ID_FLAGS(func, bpf_lsm_policy_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_lsm_policy_from_fd,
 	     KF_ACQUIRE | KF_RET_NULL | KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_lsm_policy_release, KF_RELEASE)
-- 
2.55.0