exec: avoid RLIMIT_STACK races with prlimit()

exec: avoid RLIMIT_STACK races with prlimit()

While the defense-in-depth RLIMIT_STACK limit on setuid processes was
protected against races from other threads calling setrlimit(), I missed
protecting it against races from external processes calling prlimit().
This adds locking around the change and makes sure that rlim_max is set
too.

Link: http://lkml.kernel.org/r/20171127193457.GA11348@beast
Fixes: 64701dee4178e ("exec: Use sane stack rlimit under secureexec")
Signed-off-by: Kees Cook <[email protected]>
Reported-by: Ben Hutchings <[email protected]>
Reported-by: Brad Spengler <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
Cc: James Morris <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>