KEYS: Fix race between read and revoke

This fixes CVE-2015-7550.

There's a race between keyctl_read() and keyctl_revoke().  If the revoke
happens between keyctl_read() checking the validity of a key and the key's
semaphore being taken, then the key type read method will see a revoked key.

This causes a problem for the user-defined key type because it assumes in
its read method that there will always be a payload in a non-revoked key
and doesn't check for a NULL pointer.

Fix this by making keyctl_read() check the validity of a key after taking
semaphore instead of before.

I think the bug was introduced with the original keyrings code.

This was discovered by a multithreaded test program generated by syzkaller
(http://github.com/google/syzkaller).  Here's a cleaned up version:

	#include <sys/types.h>
	#include <keyutils.h>
	#include <pthread.h>
	void *thr0(void *arg)
	{
		key_serial_t key = (unsigned long)arg;
		keyctl_revoke(key);
		return 0;
	}
	void *thr1(void *arg)
	{
		key_serial_t key = (unsigned long)arg;
		char buffer[16];
		keyctl_read(key, buffer, 16);
		return 0;
	}
	int main()
	{
		key_serial_t key = add_key("user", "%", "foo", 3, KEY_SPEC_USER_KEYRING);
		pthread_t th[5];
		pthread_create(&th[0], 0, thr0, (void *)(unsigned long)key);
		pthread_create(&th[1], 0, thr1, (void *)(unsigned long)key);
		pthread_create(&th[2], 0, thr0, (void *)(unsigned long)key);
		pthread_create(&th[3], 0, thr1, (void *)(unsigned long)key);
		pthread_join(th[0], 0);
		pthread_join(th[1], 0);
		pthread_join(th[2], 0);
		pthread_join(th[3], 0);
		return 0;
	}

Build as:

	cc -o keyctl-race keyctl-race.c -lkeyutils -lpthread

Run as:

	while keyctl-race; do :; done

as it may need several iterations to crash the kernel.  The crash can be
summarised as:

	BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
	IP: [<ffffffff81279b08>] user_read+0x56/0xa3
	...
	Call Trace:
	 [<ffffffff81276aa9>] keyctl_read_key+0xb6/0xd7
	 [<ffffffff81277815>] SyS_keyctl+0x83/0xe0
	 [<ffffffff815dbb97>] entry_SYSCALL_64_fastpath+0x12/0x6f

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: James Morris <james.l.morris@oracle.com>
SYSCALL_DEFINE5(keyctl, int, option, unsigned long, arg2, unsigned long, arg3,
		unsigned long, arg4, unsigned long, arg5)
{
	switch (option) {
	case KEYCTL_GET_KEYRING_ID:
		return keyctl_get_keyring_ID((key_serial_t) arg2,
					     (int) arg3);

	case KEYCTL_JOIN_SESSION_KEYRING:
		return keyctl_join_session_keyring((const char __user *) arg2);

	case KEYCTL_UPDATE:
		return keyctl_update_key((key_serial_t) arg2,
					 (const void __user *) arg3,
					 (size_t) arg4);

	case KEYCTL_REVOKE:
		return keyctl_revoke_key((key_serial_t) arg2);

	case KEYCTL_DESCRIBE:
		return keyctl_describe_key((key_serial_t) arg2,
					   (char __user *) arg3,
					   (unsigned) arg4);

	case KEYCTL_CLEAR:
		return keyctl_keyring_clear((key_serial_t) arg2);

	case KEYCTL_LINK:
		return keyctl_keyring_link((key_serial_t) arg2,
					   (key_serial_t) arg3);

	case KEYCTL_UNLINK:
		return keyctl_keyring_unlink((key_serial_t) arg2,
					     (key_serial_t) arg3);

	case KEYCTL_SEARCH:
		return keyctl_keyring_search((key_serial_t) arg2,
					     (const char __user *) arg3,
					     (const char __user *) arg4,
					     (key_serial_t) arg5);

	case KEYCTL_READ:
		return keyctl_read_key((key_serial_t) arg2,
				       (char __user *) arg3,
				       (size_t) arg4);

	case KEYCTL_CHOWN:
		return keyctl_chown_key((key_serial_t) arg2,
					(uid_t) arg3,
					(gid_t) arg4);

	case KEYCTL_SETPERM:
		return keyctl_setperm_key((key_serial_t) arg2,
					  (key_perm_t) arg3);

	case KEYCTL_INSTANTIATE:
		return keyctl_instantiate_key((key_serial_t) arg2,
					      (const void __user *) arg3,
					      (size_t) arg4,
					      (key_serial_t) arg5);

	case KEYCTL_NEGATE:
		return keyctl_negate_key((key_serial_t) arg2,
					 (unsigned) arg3,
					 (key_serial_t) arg4);

	case KEYCTL_SET_REQKEY_KEYRING:
		return keyctl_set_reqkey_keyring(arg2);

	case KEYCTL_SET_TIMEOUT:
		return keyctl_set_timeout((key_serial_t) arg2,
					  (unsigned) arg3);

	case KEYCTL_ASSUME_AUTHORITY:
		return keyctl_assume_authority((key_serial_t) arg2);

	case KEYCTL_GET_SECURITY:
		return keyctl_get_security((key_serial_t) arg2,
					   (char __user *) arg3,
					   (size_t) arg4);

	case KEYCTL_SESSION_TO_PARENT:
		return keyctl_session_to_parent();

	case KEYCTL_REJECT:
		return keyctl_reject_key((key_serial_t) arg2,
					 (unsigned) arg3,
					 (unsigned) arg4,
					 (key_serial_t) arg5);

	case KEYCTL_INSTANTIATE_IOV:
		return keyctl_instantiate_key_iov(
			(key_serial_t) arg2,
			(const struct iovec __user *) arg3,
			(unsigned) arg4,
			(key_serial_t) arg5);

	case KEYCTL_INVALIDATE:
		return keyctl_invalidate_key((key_serial_t) arg2);

	case KEYCTL_GET_PERSISTENT:
		return keyctl_get_persistent((uid_t)arg2, (key_serial_t)arg3);

	default:
		return -EOPNOTSUPP;
	}
}

SYSCALL_DEFINE5(keyctl, int, option, unsigned long, arg2, unsigned long, arg3,
		unsigned long, arg4, unsigned long, arg5)
{
	switch (option) {
	case KEYCTL_GET_KEYRING_ID:
		return keyctl_get_keyring_ID((key_serial_t) arg2,
					     (int) arg3);

	case KEYCTL_JOIN_SESSION_KEYRING:
		return keyctl_join_session_keyring((const char __user *) arg2);

	case KEYCTL_UPDATE:
		return keyctl_update_key((key_serial_t) arg2,
					 (const void __user *) arg3,
					 (size_t) arg4);

	case KEYCTL_REVOKE:
		return keyctl_revoke_key((key_serial_t) arg2);

	case KEYCTL_DESCRIBE:
		return keyctl_describe_key((key_serial_t) arg2,
					   (char __user *) arg3,
					   (unsigned) arg4);

	case KEYCTL_CLEAR:
		return keyctl_keyring_clear((key_serial_t) arg2);

	case KEYCTL_LINK:
		return keyctl_keyring_link((key_serial_t) arg2,
					   (key_serial_t) arg3);

	case KEYCTL_UNLINK:
		return keyctl_keyring_unlink((key_serial_t) arg2,
					     (key_serial_t) arg3);

	case KEYCTL_SEARCH:
		return keyctl_keyring_search((key_serial_t) arg2,
					     (const char __user *) arg3,
					     (const char __user *) arg4,
					     (key_serial_t) arg5);

	case KEYCTL_READ:
		return keyctl_read_key((key_serial_t) arg2,
				       (char __user *) arg3,
				       (size_t) arg4);

	case KEYCTL_CHOWN:
		return keyctl_chown_key((key_serial_t) arg2,
					(uid_t) arg3,
					(gid_t) arg4);

	case KEYCTL_SETPERM:
		return keyctl_setperm_key((key_serial_t) arg2,
					  (key_perm_t) arg3);

	case KEYCTL_INSTANTIATE:
		return keyctl_instantiate_key((key_serial_t) arg2,
					      (const void __user *) arg3,
					      (size_t) arg4,
					      (key_serial_t) arg5);

	case KEYCTL_NEGATE:
		return keyctl_negate_key((key_serial_t) arg2,
					 (unsigned) arg3,
					 (key_serial_t) arg4);

	case KEYCTL_SET_REQKEY_KEYRING:
		return keyctl_set_reqkey_keyring(arg2);

	case KEYCTL_SET_TIMEOUT:
		return keyctl_set_timeout((key_serial_t) arg2,
					  (unsigned) arg3);

	case KEYCTL_ASSUME_AUTHORITY:
		return keyctl_assume_authority((key_serial_t) arg2);

	case KEYCTL_GET_SECURITY:
		return keyctl_get_security((key_serial_t) arg2,
					   (char __user *) arg3,
					   (size_t) arg4);

	case KEYCTL_SESSION_TO_PARENT:
		return keyctl_session_to_parent();

	case KEYCTL_REJECT:
		return keyctl_reject_key((key_serial_t) arg2,
					 (unsigned) arg3,
					 (unsigned) arg4,
					 (key_serial_t) arg5);

	case KEYCTL_INSTANTIATE_IOV:
		return keyctl_instantiate_key_iov(
			(key_serial_t) arg2,
			(const struct iovec __user *) arg3,
			(unsigned) arg4,
			(key_serial_t) arg5);

	case KEYCTL_INVALIDATE:
		return keyctl_invalidate_key((key_serial_t) arg2);

	case KEYCTL_GET_PERSISTENT:
		return keyctl_get_persistent((uid_t)arg2, (key_serial_t)arg3);

	default:
		return -EOPNOTSUPP;
	}
}
