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 <[e-mail]>
Signed-off-by: Dave Hansen <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Ingo Molnar <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Juergen Gross <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: [e-mail]
Cc: Linus Torvalds <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Nadav Amit <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: Arjan van de Ven <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/mm/pti.c

···
  */
 void pti_clone_kernel_text(void)
 {
+	/*
+	 * rodata is part of the kernel image and is normally
+	 * readable on the filesystem or on the web.  But, do not
+	 * clone the areas past rodata, they might contain secrets.
+	 */
 	unsigned long start = PFN_ALIGN(_text);
-	unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);
+	unsigned long end = (unsigned long)__end_rodata_hpage_align;
 
 	if (!pti_kernel_image_global_ok())
 		return;
 
+	pr_debug("mapping partial kernel image into user address space\n");
+
+	/*
+	 * Note that this will undo _some_ of the work that
+	 * pti_set_kernel_image_nonglobal() did to clear the
+	 * global bit.
+	 */
 	pti_clone_pmds(start, end, _PAGE_RW);
 }
 
···
 
 	if (pti_kernel_image_global_ok())
 		return;
-
-	pr_debug("set kernel image non-global\n");
 
 	set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);
 }