x86/pti: Reduce amount of kernel text allowed to be Global

commit abb67605203687c8b7943d760638d0301787f8d9

Kees reported to me that I made too much of the kernel image global.
It was far more than just text:

	I think this is too much set global: _end is after data,
	bss, and brk, and all kinds of other stuff that could
	hold secrets. I think this should match what
	mark_rodata_ro() is doing.

This does exactly that.  We use __end_rodata_hpage_align as our
marker both because it is huge-page-aligned and it does not contain
any sections we expect to hold secrets.

Kees's logic was that r/o data is in the kernel image anyway and,
in the case of traditional distributions, can be freely downloaded
from the web, so there's no reason to hide it.

Fixes: 8c06c7740 (x86/pti: Leave kernel text global for !PCID)
Reported-by: Kees Cook <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Ingo Molnar <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: [email protected]
Cc: Linus Torvalds <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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