[PATCH] x86/pti: unpoison pgd for trusted boot

From: Dave Hansen
Date: Wed Jan 10 2018 - 16:13:29 EST



I believe this should replace 262b6b30087 in -tip.

The patch in -tip potentially misses the pgd clearing if pud_alloc()
sets a PGD. It would also be nice to have that comment back.

Note that the -tip commit probably works in *practice* because for
two adjacent calls to map_tboot_page() that share a PGD entry, the
first will clear NX, *then* allocate and set the PGD (without NX
clear). The second call will *not* allocate but will clear the NX
bit.

--

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

This is another case similar to what EFI does: create a new set of
page tables, map some code at a low address, and jump to it. PTI
mistakes this low address for userspace and mistakenly marks it
non-executable in an effort to make it unusable for userspace. Undo
the poison to allow execution.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Ning Sun <ning.sun@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: tboot-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
CC: Jon Masters <jcm@xxxxxxxxxx>
Cc: "Woodhouse, David" <dwmw@xxxxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxxx>
CC: "Tim Chen" <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Jeff Law <law@xxxxxxxxxx>
Cc: Nick Clifton <nickc@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---

b/arch/x86/kernel/tboot.c | 11 +++++++++++
1 file changed, 11 insertions(+)

diff -puN arch/x86/kernel/tboot.c~pti-tboot-fix arch/x86/kernel/tboot.c
--- a/arch/x86/kernel/tboot.c~pti-tboot-fix 2018-01-09 17:12:49.776734656 -0800
+++ b/arch/x86/kernel/tboot.c 2018-01-09 17:12:49.784734656 -0800
@@ -138,6 +138,17 @@ static int map_tboot_page(unsigned long
return -1;
set_pte_at(&tboot_mm, vaddr, pte, pfn_pte(pfn, prot));
pte_unmap(pte);
+
+ /*
+ * PTI poisons low addresses in the kernel page tables in the
+ * name of making them unusable for userspace. To execute
+ * code at such a low address, the poison must be cleared.
+ *
+ * Note: 'pgd' actually gets set in p4d_alloc() _or_
+ * pud_alloc() depending on 4/5-level paging.
+ */
+ pgd->pgd &= ~_PAGE_NX;
+
return 0;
}

_