bpf: Refuse caller-supplied keyrings when the bpf one is active

Nothing changes for systems that do not use the bpf keyring. Without
bpf.keyring_unsealed=1 a caller-supplied keyring behaves exactly as
before, which also lets it serve as the staging step for software
installed onto a running system whose signing key is not enrolled
anywhere yet.

Passing bpf.keyring_unsealed=1 states that the bpf keyring is the trust
anchor for this boot, so from the first program load onwards a caller-
supplied keyring is refused with -EPERM. Deriving this from the boot
flag rather than from the keyring's runtime state keeps the decision
immutable from userspace.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260828175227.1537793-3-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -673,6 +673,14 @@ Kernel parameters
 			keys. Once the keyring is restricted it becomes active
 			and can be used for BPF program signature verification.
 
+			Setting this also means that the bpf keyring is the
+			only non-system keyring a loader may select for the
+			rest of the boot: caller-supplied user/session
+			keyrings are refused with -EPERM, whether or not
+			provisioning actually completed. The system keyrings
+			stay selectable. Leaving it unset keeps the prior
+			behaviour, where a caller-supplied keyring is allowed.
+
 			See Documentation/bpf/signing.rst
 
 	bttv.card=	[HW,V4L] bttv (bt848 + bt878 based grabber cards)

include/linux/bpf.h

@@ -3821,6 +3821,7 @@ struct bpf_key {
 struct bpf_key *bpf_lookup_user_key(s32 serial, u64 flags);
 struct bpf_key *bpf_lookup_system_key(u64 id);
 struct bpf_key *bpf_lookup_keyring(void);
+bool bpf_keyring_enforced(void);
 void bpf_key_put(struct bpf_key *bkey);
 int bpf_verify_pkcs7_signature(const struct bpf_dynptr *data_p,
 			       const struct bpf_dynptr *sig_p,
@@ -3846,6 +3847,11 @@ static inline struct bpf_key *bpf_lookup_keyring(void)
 	return NULL;
 }
 
+static inline bool bpf_keyring_enforced(void)
+{
+	return false;
+}
+
 static inline void bpf_key_put(struct bpf_key *bkey)
 {
 }

kernel/bpf/keys.c

@@ -18,6 +18,11 @@ static bool bpf_keyring_unsealed __ro_after_init;
 module_param_named(keyring_unsealed, bpf_keyring_unsealed, bool, 0444);
 MODULE_PARM_DESC(keyring_unsealed, "Leave the bpf keyring unsealed");
 
+bool bpf_keyring_enforced(void)
+{
+	return bpf_keyring_unsealed;
+}
+
 struct bpf_key *bpf_lookup_keyring(void)
 {
 	struct bpf_key *bkey;

kernel/bpf/verifier.c

@@ -21165,12 +21165,16 @@ static int bpf_prog_verify_signature(struct bpf_verifier_env *env,
 	if (!attr->signature_size ||
 	    attr->signature_size > KMALLOC_MAX_CACHE_SIZE)
 		return -EINVAL;
-	if (system_keyring_id_check(attr->keyring_id) == 0)
+	if (system_keyring_id_check(attr->keyring_id) == 0) {
 		key = bpf_lookup_system_key(attr->keyring_id);
-	else if (attr->keyring_id == KEY_SPEC_BPF_KEYRING)
+	} else if (attr->keyring_id == KEY_SPEC_BPF_KEYRING) {
 		key = bpf_lookup_keyring();
-	else
+	} else if (bpf_keyring_enforced()) {
+		verbose(env, "caller-supplied keyring refused, use bpf keyring\n");
+		return -EPERM;
+	} else {
 		key = bpf_lookup_user_key(attr->keyring_id, 0);
+	}
 	if (!key) {
 		if (attr->keyring_id == KEY_SPEC_BPF_KEYRING) {
 			verbose(env, "the bpf keyring is empty or has not been restricted\n");