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 <[email protected]>
Reported-by: Kees Cook <[email protected]>
Fixes: 8c06c7740 (x86/pti: Leave kernel text global for !PCID)
Cc: Andrea Arcangeli <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
---
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);
}
_
