keys: Protect the read method check against type removal

keyctl_read_key() checks key->type->read without holding key->sem.
A reference keeps the key alive, but does not pin the module containing
its type. A surviving RxRPC key can therefore be read while AF_RXRPC
is being unloaded:

  KEYCTL_READ task                    Module unload and key GC
  ----------------                   ------------------------
  lookup_user_key()
  permission / possession checks
  load key->type
                                     af_rxrpc_exit()
                                       unregister_key_type()
                                         GC: down_write(key->sem)
                                         GC: retype to key_type_dead
                                         GC: up_write(key->sem)
                                       unregister_key_type() returns
                                     free_module()
  read saved type's read member

The final access can read freed module storage. The read invocation in
__keyctl_read_key() is already protected by key->sem, but the earlier
method check is outside that critical section.

Move the method check into __keyctl_read_key(), after key_validate() and
under its existing read hold on key->sem. The check and invocation then
use the same protected type for length queries and every read retry.
The GC cannot retype the key until the semaphore is released, and type
unregistration waits for that GC pass before the module can be freed.

Return -EOPNOTSUPP when validation succeeds and the type has no read
method. A temporary buffer may now be allocated before this result;
allocation and copying to userspace remain outside the semaphore.

Assisted-by: LLM
Cc: stable@vger.kernel.org # v5.10+
Fixes: d3ec10aa9581 ("KEYS: Don't write out to userspace while holding key semaphore")
Suggested-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/pm-key-management-objects-candidate-0010-v7-fc85721ff5cfb9e33ac6@gmail.com
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

1 files changed, 6 insertions, 7 deletions

diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 5a5efa26ed0e..36ffba0ebba6 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -808,8 +808,12 @@ static long __keyctl_read_key(struct key *key, char *buffer, size_t buflen)
 
 	down_read(&key->sem);
 	ret = key_validate(key);
-	if (ret == 0)
-		ret = key->type->read(key, buffer, buflen);
+	if (ret == 0) {
+		if (key->type->read)
+			ret = key->type->read(key, buffer, buflen);
+		else
+			ret = -EOPNOTSUPP;
+	}
 	up_read(&key->sem);
 	return ret;
 }
@@ -863,11 +867,6 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
 
 	/* the key is probably readable - now try to read it */
 can_read_key:
-	if (!key->type->read) {
-		ret = -EOPNOTSUPP;
-		goto key_put_out;
-	}
-
 	if (!buffer || !buflen) {
 		/* Get the key length from the read method */
 		ret = __keyctl_read_key(key, NULL, 0);