KEYS: keyctl_get_keyring_ID() should create a session keyring if create flag set

The keyctl call:

	keyctl_get_keyring_ID(KEY_SPEC_SESSION_KEYRING, 1)

should create a session keyring if the process doesn't have one of its own
because the create flag argument is set - rather than subscribing to and
returning the user-session keyring as:

	keyctl_get_keyring_ID(KEY_SPEC_SESSION_KEYRING, 0)

will do.

This can be tested by commenting out pam_keyinit in the /etc/pam.d files and
running the following program a couple of times in a row:

	#include <stdio.h>
	#include <stdlib.h>
	#include <keyutils.h>
	int main(int argc, char *argv[])
	{
		key_serial_t uk, usk, sk, nsk;
		uk  = keyctl_get_keyring_ID(KEY_SPEC_USER_KEYRING, 0);
		usk = keyctl_get_keyring_ID(KEY_SPEC_USER_SESSION_KEYRING, 0);
		sk  = keyctl_get_keyring_ID(KEY_SPEC_SESSION_KEYRING, 0);
		nsk = keyctl_get_keyring_ID(KEY_SPEC_SESSION_KEYRING, 1);
		printf("keys: %08x %08x %08x %08x\n", uk, usk, sk, nsk);
		return 0;
	}

Without this patch, I see:

	keys: 3975ddc7 119c0c66 119c0c66 119c0c66
	keys: 3975ddc7 119c0c66 119c0c66 119c0c66

With this patch, I see:

	keys: 2cb4997b 34112878 34112878 17db2ce3
	keys: 2cb4997b 34112878 34112878 39f3c73e

As can be seen, the session keyring starts off the same as the user-session
keyring each time, but with the patch a new session keyring is created when
the create flag is set.
Reported-by: Greg Wettstein <greg@enjellic.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Greg Wettstein <greg@enjellic.com>
Signed-off-by: James Morris <jmorris@namei.org>

security/keys/process_keys.c

@@ -589,12 +589,22 @@ key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
 			ret = install_user_keyrings();
 			if (ret < 0)
 				goto error;
+			if (lflags & KEY_LOOKUP_CREATE)
+				ret = join_session_keyring(NULL);
+			else
 				ret = install_session_keyring(
 					cred->user->session_keyring);
 
 			if (ret < 0)
 				goto error;
 			goto reget_creds;
+		} else if (cred->tgcred->session_keyring ==
+			   cred->user->session_keyring &&
+			   lflags & KEY_LOOKUP_CREATE) {
+			ret = join_session_keyring(NULL);
+			if (ret < 0)
+				goto error;
+			goto reget_creds;
 		}
 
 		rcu_read_lock();