BACKPORT: make 'user_access_begin()' do 'access_ok()'

upstream commit 594cc251 ("make 'user_access_begin()' do 'access_ok()'")

Originally, the rule used to be that you'd have to do access_ok()
separately, and then user_access_begin() before actually doing the
direct (optimized) user access.

But the experience has shown that people then decide not to do access_ok()
at all, and instead rely on it being implied by other operations or
similar.  Which makes it very hard to verify that the access has
actually been range-checked.

If you use the unsafe direct user accesses, hardware features (either
SMAP - Supervisor Mode Access Protection - on x86, or PAN - Privileged
Access Never - on ARM) do force you to use user_access_begin().  But
nothing really forces the range check.

By putting the range check into user_access_begin(), we actually force
people to do the right thing (tm), and the range check vill be visible
near the actual accesses.  We have way too long a history of people
trying to avoid them.

Bug: 135368228
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git


Git-commit: 216284c4
Change-Id: I158cd79c9c05fdf33caf8a105fc43564493dc96d
Signed-off-by: Kishor Krishna Bhat <quic_kishkris@quicinc.com>