keys: add KEY_SPEC_DM_VERITY_KEYRING

The .dm-verity keyring is not linked into any process keyring, so
request_key() and KEYCTL_SEARCH cannot find it. Userspace has to scrape
/proc/keys for the serial to use.

That's not great, one could put in a bogus description containing
a newline to confuse userspace further. For example, thanks to AI
models, which I run as root but you could show off as any old user:

    # DESC=$'.dm-verity: 1\n0badf00d I--Q---     1 perm 082f0000     0     0 keyring   .dm-verity'
    # k=$(echo -n "1234567" | keyctl padd user "$DESC" @s)
    # keyctl setperm "$k" 0x3f010001

    # cat /proc/keys | grep dm-verity
    10402023 I------     2 perm 082f0000     0     0 keyring   .dm-verity: empty
    12e0bee6 I--Q---     1 perm 3f010001     0     0 user      .dm-verity: 1
    0badf00d I--Q---     1 perm 082f0000     0     0 keyring   .dm-verity: 7

0x0badf00d is not a keyring, it is the text inside our user's key
description. Telling the two apart needs a KEYCTL_DESCRIBE per row
which is annoying to do. i.e.:

    # keyctl rdescribe 0x10402023
    keyring;0;0;082f0000;.dm-verity
    # keyctl rdescribe 0x0badf00d
    keyctl_describe: Required key not available

Let's just give it a well known identifier, i.e.:

    # keyctl rdescribe -10
    keyring;0;0;082f0000;.dm-verity

This is exactly what
commit 264d8fd279 ("bpf, keys: Add a bpf keyring for program signature validation")
did, and was the inspiration for this commit minus the spinlock bits
required due to dm-verity being a module.

Suggested-by: Christian Brauner (Amutable) <brauner@kernel.org>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Andrew Halaney <andrew@amutable.com>
Link: https://patch.msgid.link/20261008-ajhalaney-dmverity-key-identifier-v2-1-008456d189cb@amutable.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

drivers/md/dm-verity-verify-sig.c

@@ -189,11 +189,14 @@ int __init dm_verity_verify_sig_init(void)
 	    keyring_restrict(make_key_ref(dm_verity_keyring, true), NULL, NULL))
 		panic("dm-verity can't seal keyring\n");
 
+	key_register_dm_verity_keyring(dm_verity_keyring);
+
 	return 0;
 }
 
 void dm_verity_verify_sig_exit(void)
 {
+	key_register_dm_verity_keyring(NULL);
 	key_revoke(dm_verity_keyring);
 	key_put(dm_verity_keyring);
 }

include/linux/key.h

@@ -441,6 +441,7 @@ extern int keyring_restrict(key_ref_t keyring, const char *type,
 			    const char *restriction);
 
 extern void key_register_bpf_keyring(struct key *keyring);
+extern void key_register_dm_verity_keyring(struct key *keyring);
 
 extern struct key *key_lookup(key_serial_t id);
 

include/uapi/linux/keyctl.h

@@ -25,6 +25,7 @@
 #define KEY_SPEC_REQKEY_AUTH_KEY	-7	/* - key ID for assumed request_key auth key */
 #define KEY_SPEC_REQUESTOR_KEYRING	-8	/* - key ID for request_key() dest keyring */
 #define KEY_SPEC_BPF_KEYRING		-9	/* - key ID for the BPF-specific keyring */
+#define KEY_SPEC_DM_VERITY_KEYRING	-10	/* - key ID for the .dm-verity keyring */
 
 /* request-key default keyrings */
 #define KEY_REQKEY_DEFL_NO_CHANGE		-1

security/keys/process_keys.c

@@ -25,6 +25,10 @@ static DEFINE_MUTEX(key_session_mutex);
 /* BPF keyring reachable through KEY_SPEC_BPF_KEYRING */
 static struct key *bpf_keyring __ro_after_init;
 
+/* dm-verity keyring reachable through KEY_SPEC_DM_VERITY_KEYRING */
+static struct key *dm_verity_keyring;
+static DEFINE_SPINLOCK(dm_verity_keyring_lock);
+
 /* The root user's tracking struct */
 struct key_user root_key_user = {
 	.usage		= REFCOUNT_INIT(3),
@@ -607,6 +611,24 @@ void key_register_bpf_keyring(struct key *keyring)
 	bpf_keyring = keyring;
 }
 
+/**
+ * key_register_dm_verity_keyring - Publish the keyring for KEY_SPEC_DM_VERITY_KEYRING
+ * @keyring: The keyring to publish, or NULL to withdraw it
+ *
+ * Make @keyring reachable by userspace through the KEY_SPEC_DM_VERITY_KEYRING
+ * special key ID, so that provisioning it does not require scraping its
+ * serial out of /proc/keys first. dm-verity can be unloaded as a module
+ * and the keyring deregistered, as such serialize access with
+ * dm_verity_keyring_lock.
+ */
+void key_register_dm_verity_keyring(struct key *keyring)
+{
+	spin_lock(&dm_verity_keyring_lock);
+	dm_verity_keyring = keyring;
+	spin_unlock(&dm_verity_keyring_lock);
+}
+EXPORT_SYMBOL_GPL(key_register_dm_verity_keyring);
+
 /*
  * Look up a key ID given us by userspace with a given permissions mask to get
  * the key it refers to.
@@ -766,6 +788,17 @@ key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
 		key_ref = make_key_ref(key, 0);
 		break;
 
+	case KEY_SPEC_DM_VERITY_KEYRING:
+		spin_lock(&dm_verity_keyring_lock);
+		key = dm_verity_keyring;
+		if (key)
+			__key_get(key);
+		spin_unlock(&dm_verity_keyring_lock);
+		if (!key)
+			goto error;
+		key_ref = make_key_ref(key, 0);
+		break;
+
 	default:
 		key_ref = ERR_PTR(-EINVAL);
 		if (id < 1)