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.han...@linux.intel.com> 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.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 | 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; } _