keys: Pin request_key_auth payload in instantiate paths

commit fd15b45 upstream.

A: request_key()       B: KEYCTL_INSTANTIATE_IOV
================       =========================

create auth key
store rka in auth key
wait for helper
                       get auth key
                       load rka from auth key
                       copy user payload
                       sleep on #PF

helper completed
detach and free rka
destroy auth key
                       wake up
                       use rka->target_key
                       **USE-AFTER-FREE**

Give request_key_auth payloads a refcount.  Take a payload reference while
authkey->sem stabilizes the payload and revocation state.  Hold that
reference across the instantiate and reject paths.  Drop the auth key
owning reference from revoke and destroy.

[jarkko: Replaced the first two paragraphs of text with an actual
 concurrency scenario.]
Cc: stable@vger.kernel.org # v5.10+
Fixes: b5f545c ("[PATCH] keys: Permit running process to instantiate keys")
Reported-by: Shaomin Chen <eeesssooo020@gmail.com>
Closes: https://lore.kernel.org/r/20260519144403.436694-1-eeesssooo020@gmail.com
Signed-off-by: Shaomin Chen <eeesssooo020@gmail.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: CsomePro

include/keys/request_key_auth-type.h

@@ -9,12 +9,14 @@
 #define _KEYS_REQUEST_KEY_AUTH_TYPE_H
 
 #include <linux/key.h>
+#include <linux/refcount.h>
 
 /*
  * Authorisation record for request_key().
  */
 struct request_key_auth {
 	struct rcu_head		rcu;
+	refcount_t		usage;
 	struct key		*target_key;
 	struct key		*dest_keyring;
 	const struct cred	*cred;

security/keys/internal.h

@@ -208,6 +208,8 @@ extern struct key *request_key_auth_new(struct key *target,
 					const void *callout_info,
 					size_t callout_len,
 					struct key *dest_keyring);
+struct request_key_auth *request_key_auth_get(struct key *authkey);
+void request_key_auth_put(struct request_key_auth *rka);
 
 extern struct key *key_get_instantiation_authkey(key_serial_t target_id);
 

security/keys/keyctl.c

@@ -1197,9 +1197,13 @@ static long keyctl_instantiate_key_common(key_serial_t id,
 	if (!instkey)
 		goto error;
 
-	rka = instkey->payload.data[0];
-	if (rka->target_key->serial != id)
+	rka = request_key_auth_get(instkey);
+	if (!rka) {
+		ret = -EKEYREVOKED;
 		goto error;
+	}
+	if (rka->target_key->serial != id)
+		goto error_put_rka;
 
 	/* pull the payload in if one was supplied */
 	payload = NULL;
@@ -1208,7 +1212,7 @@ static long keyctl_instantiate_key_common(key_serial_t id,
 		ret = -ENOMEM;
 		payload = kvmalloc(plen, GFP_KERNEL);
 		if (!payload)
-			goto error;
+			goto error_put_rka;
 
 		ret = -EFAULT;
 		if (!copy_from_iter_full(payload, plen, from))
@@ -1234,6 +1238,8 @@ static long keyctl_instantiate_key_common(key_serial_t id,
 
 error2:
 	kvfree_sensitive(payload, plen);
+error_put_rka:
+	request_key_auth_put(rka);
 error:
 	return ret;
 }
@@ -1358,15 +1364,19 @@ long keyctl_reject_key(key_serial_t id, unsigned timeout, unsigned error,
 	if (!instkey)
 		goto error;
 
-	rka = instkey->payload.data[0];
-	if (rka->target_key->serial != id)
+	rka = request_key_auth_get(instkey);
+	if (!rka) {
+		ret = -EKEYREVOKED;
 		goto error;
+	}
+	if (rka->target_key->serial != id)
+		goto error_put_rka;
 
 	/* find the destination keyring if present (which must also be
 	 * writable) */
 	ret = get_instantiation_keyring(ringid, rka, &dest_keyring);
 	if (ret < 0)
-		goto error;
+		goto error_put_rka;
 
 	/* instantiate the key and link it into a keyring */
 	ret = key_reject_and_link(rka->target_key, timeout, error,
@@ -1379,6 +1389,8 @@ long keyctl_reject_key(key_serial_t id, unsigned timeout, unsigned error,
 	if (ret == 0)
 		keyctl_change_reqkey_auth(NULL);
 
+error_put_rka:
+	request_key_auth_put(rka);
 error:
 	return ret;
 }

security/keys/request_key_auth.c

@@ -23,6 +23,7 @@ static void request_key_auth_describe(const struct key *, struct seq_file *);
 static void request_key_auth_revoke(struct key *);
 static void request_key_auth_destroy(struct key *);
 static long request_key_auth_read(const struct key *, char *, size_t);
+static void request_key_auth_rcu_disposal(struct rcu_head *);
 
 /*
  * The request-key authorisation key type definition.
@@ -115,6 +116,31 @@ static void free_request_key_auth(struct request_key_auth *rka)
 	kfree(rka);
 }
 
+/*
+ * Take a reference to the request-key authorisation payload so callers can
+ * drop authkey->sem before doing operations that may sleep.
+ */
+struct request_key_auth *request_key_auth_get(struct key *authkey)
+{
+	struct request_key_auth *rka;
+
+	down_read(&authkey->sem);
+	rka = dereference_key_locked(authkey);
+	if (rka && !test_bit(KEY_FLAG_REVOKED, &authkey->flags))
+		refcount_inc(&rka->usage);
+	else
+		rka = NULL;
+	up_read(&authkey->sem);
+
+	return rka;
+}
+
+void request_key_auth_put(struct request_key_auth *rka)
+{
+	if (rka && refcount_dec_and_test(&rka->usage))
+		call_rcu(&rka->rcu, request_key_auth_rcu_disposal);
+}
+
 /*
  * Dispose of the request_key_auth record under RCU conditions
  */
@@ -136,8 +162,10 @@ static void request_key_auth_revoke(struct key *key)
 	struct request_key_auth *rka = dereference_key_locked(key);
 
 	kenter("{%d}", key->serial);
+	if (!rka)
+		return;
 	rcu_assign_keypointer(key, NULL);
-	call_rcu(&rka->rcu, request_key_auth_rcu_disposal);
+	request_key_auth_put(rka);
 }
 
 /*
@@ -150,7 +178,7 @@ static void request_key_auth_destroy(struct key *key)
 	kenter("{%d}", key->serial);
 	if (rka) {
 		rcu_assign_keypointer(key, NULL);
-		call_rcu(&rka->rcu, request_key_auth_rcu_disposal);
+		request_key_auth_put(rka);
 	}
 }
 
@@ -174,6 +202,7 @@ struct key *request_key_auth_new(struct key *target, const char *op,
 	rka = kzalloc(sizeof(*rka), GFP_KERNEL);
 	if (!rka)
 		goto error;
+	refcount_set(&rka->usage, 1);
 	rka->callout_info = kmemdup(callout_info, callout_len, GFP_KERNEL);
 	if (!rka->callout_info)
 		goto error_free_rka;