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 <[email protected]> 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 <[email protected]> Cc: Ning Sun <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: "H. Peter Anvin" <[email protected]> Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: Andrea Arcangeli <[email protected]> CC: Jon Masters <[email protected]> Cc: "Woodhouse, David" <[email protected]> Cc: Paolo Bonzini <[email protected]> Cc: Alan Cox <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Andi Kleen <[email protected]> Cc: Greg Kroah-Hartman <[email protected]> CC: "Tim Chen" <[email protected]> Cc: Jeff Law <[email protected]> Cc: Nick Clifton <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Peter Zijlstra <[email protected]> --- 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; } _
