fork,random: use get_random_canary() to set tsk->stack_canary

Use the ascii-armor canary to prevent unterminated C string overflows
from being able to successfully overwrite the canary, even if they
somehow obtain the canary value.

Inspired by execshield ascii-armor and Daniel Micay's linux-hardened
tree.

Link: http://lkml.kernel.org/r/20170524155751.424-3-riel@redhat.com


Change-Id: Ida81572d9ff478551283435e26255dc2c1b9ec17
Signed-off-by: Rik van Riel <riel@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Chenyang Zhong <zhongcy95@gmail.com>
(cherry picked from commit 00185f9fd010dc5e909fbfe04e411c806dc2c71b)
[kras: while what's supposed to be improved here already exists in part on QUIC
       kernel branches, I decided that this should be on a11/upstream-aosp
       anyway due to fact that the former has been included as part of 4.9.320]
Signed-off-by: Albert I <kras@raphielgang.org>
(cherry picked from commit d87062b547f4f1d44f7ab42ba80cfbeadc0152a1)