On Tue, 9 Jan 2018, Dave Hansen wrote:
> On 01/09/2018 04:45 PM, Thomas Gleixner wrote:
> > On Mon, 8 Jan 2018, Andrea Arcangeli wrote:
> >> On Mon, Jan 08, 2018 at 09:53:02PM +0100, Thomas Gleixner wrote:
> >> Did my best to do the cleanest patch for tip, but I now figured Dave's
> >> original comment was spot on: a _PAGE_NX clear then becomes necessary
> >> also after pud_alloc not only after p4d_alloc.
> >>
> >> pmd_alloc would run into the same with x86 32bit non-PAE too.
>
> non-PAE doesn't have an NX bit. :)
>
> But we #define _PAGE_NX down to 0 there so it's harmless.
>
> >> So there are two choices, either going back to one single _PAGE_NX
> >> clear from the original Dave's original patch as below, or to add
> >> multiple clear after each level which was my objective and is more
> >> robust, but it may be overkill in this case. As long as it was one
> >> line it looked a clear improvement.
> >>
> >> Considering the caller in both cases is going to abort I guess we can
> >> use the one liner approach as Dave and Jiri did originally.
> >
> > Dave ?
>
> I agree with Andrea. 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.
>
> The patch I think we want is attached.
Color me confused. I have queued the one below in tip. It lacks the comment
and does the !NX at a different place.
Thanks,
tglx
8<-----------------
commit 262b6b30087246abf09d6275eb0c0dc421bcbe38
Author: Dave Hansen <[email protected]>
Date: Sat Jan 6 18:41:14 2018 +0100
x86/tboot: Unbreak tboot with PTI enabled
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.
Fixes: 385ce0ea4c07 ("x86/mm/pti: Add Kconfig")
Signed-off-by: Dave Hansen <[email protected]>
Signed-off-by: Andrea Arcangeli <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Alan Cox <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Jon Masters <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Jeff Law <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: David" <[email protected]>
Cc: Nick Clifton <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]
diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
index a4eb27918ceb..75869a4b6c41 100644
--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -127,6 +127,7 @@ static int map_tboot_page(unsigned long vaddr, unsigned
long pfn,
p4d = p4d_alloc(&tboot_mm, pgd, vaddr);
if (!p4d)
return -1;
+ pgd->pgd &= ~_PAGE_NX;
pud = pud_alloc(&tboot_mm, p4d, vaddr);
if (!pud)
return -1;
