[PATCH 3/5] x86, pti: reduce amount of kernel text allowed to be Global

[Dave Hansen]

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.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reported-by: Kees Cook <keescook@xxxxxxxxxx>
Fixes: 8c06c7740 (x86/pti: Leave kernel text global for !PCID)
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Nadav Amit <namit@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
---

 b/arch/x86/mm/pti.c |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff -puN arch/x86/mm/pti.c~pti-glb-too-much-mapped arch/x86/mm/pti.c
--- a/arch/x86/mm/pti.c~pti-glb-too-much-mapped	2018-04-20 14:10:02.164749166 -0700
+++ b/arch/x86/mm/pti.c	2018-04-20 14:10:02.168749166 -0700
@@ -430,12 +430,24 @@ static inline bool pti_kernel_image_glob
  */
 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);
 }
 
@@ -458,8 +470,6 @@ void pti_set_kernel_image_nonglobal(void
 	if (pti_kernel_image_global_ok())
 		return;
 
-	pr_debug("set kernel image non-global\n");
-
 	set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);
 }
 
_