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.
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] --- 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 2018-01-05 21:50:55.755554960 -0800 +++ b/arch/x86/kernel/tboot.c 2018-01-05 23:51:41.368536890 -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; } _
