dlm: don't leak kernel pointer to userspace

[ Upstream commit 9de30f3f7f4d31037cfbb7c787e1089c1944b3a7 ]

In copy_result_to_user(), we first create a struct dlm_lock_result, which
contains a struct dlm_lksb, the last member of which is a pointer to the
lvb. Unfortunately, we copy the entire struct dlm_lksb to the result
struct, which is then copied to userspace at the end of the function,
leaking the contents of sb_lvbptr, which is a valid kernel pointer in some
cases (indeed, later in the same function the data it points to is copied
to userspace).

It is an error to leak kernel pointers to userspace, as it undermines KASLR
protections (see e.g. 65eea8ed ("floppy: Do not copy a kernel pointer to
user memory in FDGETPRM ioctl") for another example of this).

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Signed-off-by: David Teigland <teigland@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>