Updated to make this on top of x86/pti.
--
From: Dave Hansen <dave.han...@linux.intel.com>
The code 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.
This just defers the NX clearing to a point after it is known that
all top-level allocations have occurred. Add a comment to clarify
why.
Fixes: 262b6b30087 ("x86/tboot: Unbreak tboot with PTI enabled")
Signed-off-by: Dave Hansen <dave.han...@linux.intel.com>
Cc: Ning Sun <ning....@intel.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: "H. Peter Anvin" <h...@zytor.com>
Cc: x...@kernel.org
Cc: tboot-de...@lists.sourceforge.net
Cc: linux-kernel@vger.kernel.org
Cc: Andrea Arcangeli <aarca...@redhat.com>
CC: Jon Masters <j...@redhat.com>
Cc: "Woodhouse, David" <d...@amazon.co.uk>
Cc: Paolo Bonzini <pbonz...@redhat.com>
Cc: Alan Cox <gno...@lxorguk.ukuu.org.uk>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Andi Kleen <a...@firstfloor.org>
Cc: Greg Kroah-Hartman <gre...@linux-foundation.org>
CC: "Tim Chen" <tim.c.c...@linux.intel.com>
Cc: Jeff Law <l...@redhat.com>
Cc: Nick Clifton <ni...@redhat.com>
Cc: Andy Lutomirski <l...@kernel.org>
Cc: Peter Zijlstra <pet...@infradead.org>
---
b/arch/x86/kernel/tboot.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff -puN arch/x86/kernel/tboot.c~pti-tboot-fix arch/x86/kernel/tboot.c
--- a/arch/x86/kernel/tboot.c~pti-tboot-fix 2018-01-10 14:24:46.454544324
-0800
+++ b/arch/x86/kernel/tboot.c 2018-01-10 14:25:53.354544157 -0800
@@ -127,7 +127,6 @@ static int map_tboot_page(unsigned long
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;
@@ -139,6 +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;
}
_
