BACKPORT: lib: introduce copy_struct_from_user() helper

(cherry picked from commit f5a1a536fa14895ccff4e94e6a5af90901ce86aa)

A common pattern for syscall extensions is increasing the size of a
struct passed from userspace, such that the zero-value of the new fields
result in the old kernel behaviour (allowing for a mix of userspace and
kernel vintages to operate on one another in most cases).

While this interface exists for communication in both directions, only
one interface is straightforward to have reasonable semantics for
(userspace passing a struct to the kernel). For kernel returns to
userspace, what the correct semantics are (whether there should be an
error if userspace is unaware of a new extension) is very
syscall-dependent and thus probably cannot be unified between syscalls
(a good example of this problem is [1]).

Previously there was no common lib/ function that implemented
the necessary extension-checking semantics (and different syscalls
implemented them slightly differently or incompletely[2]). Future
patches replace common uses of this pattern to make use of
copy_struct_from_user().

Some in-kernel selftests that insure that the handling of alignment and
various byte patterns are all handled identically to memchr_inv() usage.
[1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI issue, clean up and
         robustify sched_read_attr() ABI logic and code").

[2]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
         similar checks to copy_struct_from_user() while rt_sigprocmask(2)
         always rejects differently-sized struct arguments.

Bug: 261681571
Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191001011055.19283-2-cyphar@cyphar.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Git-Commit: f5a1a536fa14895ccff4e94e6a5af90901ce86aa
Git-Repo: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git


Change-Id: Iea0cfb0efd37282daaf1fe2b45b4c1abd4e57adf
Signed-off-by: Kishor Krishna Bhat <quic_kishkris@quicinc.com>
Signed-off-by: JohnnLee <johnnlee@google.com>