4.9-stable review patch. If anyone has any objections, please let me know.
------------------
From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
commit 445b69e3b75e42362a5bdc13c8b8f61599e2228a upstream
The inital fix for trusted boot and PTI potentially misses the pgd clearing
if pud_alloc() sets a PGD. It 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.
Defer the NX clearing to a point after it is known that all top-level
allocations have occurred. Add a comment to clarify why.
[ tglx: Massaged changelog ]
[ hughd notes: I have not tested tboot, but this looks to me as necessary
and as safe in old-Kaiser backports as it is upstream; I'm not submitting
the commit-to-be-fixed 262b6b30087, since it was undone by 445b69e3b75e,
and makes conflict trouble because of 5-level's p4d versus 4-level's pgd.]
Fixes: 262b6b30087 ("x86/tboot: Unbreak tboot with PTI enabled")
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Jon Masters <jcm@xxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: gnomes@xxxxxxxxxxxxxxxxxxx
Cc: peterz@xxxxxxxxxxxxx
Cc: ning.sun@xxxxxxxxx
Cc: tboot-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: andi@xxxxxxxxxxxxxx
Cc: luto@xxxxxxxxxx
Cc: law@xxxxxxxxxx
Cc: pbonzini@xxxxxxxxxx
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx
Cc: gregkh@xxxxxxxxxxxxxxxxxxxx
Cc: dwmw@xxxxxxxxxxxx
Cc: nickc@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20180110224939.2695CD47@xxxxxxxxxxxxxxxxxx
Cc: Jiri Kosina <jkosina@xxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kernel/tboot.c | 10 ++++++++++
1 file changed, 10 insertions(+)
--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -134,6 +134,16 @@ 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 pud_alloc().
+ */
+ pgd->pgd &= ~_PAGE_NX;
+
return 0;
}