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

From: tip-bot for Dave Hansen
Date: Wed Apr 25 2018 - 05:09:03 EST

Commit-ID: a44ca8f5a30c008b54d07b00eed4eae7f169fcd0
Gitweb: https://git.kernel.org/tip/a44ca8f5a30c008b54d07b00eed4eae7f169fcd0
Author: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
AuthorDate: Fri, 20 Apr 2018 15:20:23 -0700
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Wed, 25 Apr 2018 11:02:50 +0200

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 <keescook@xxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Nadav Amit <namit@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20180420222023.1C8B2B20@xxxxxxxxxxxxxxxxxx

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

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index f1fd52f449e0..ae3eb4f5d53b 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -430,12 +430,24 @@ static inline bool pti_kernel_image_global_ok(void)
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())

+ 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())

- pr_debug("set kernel image non-global\n");
set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);