On 01/05/2018 10:53 PM, Hanjun Guo wrote: >> + /* >> + * 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. >> + */ >> + pgd->pgd &= ~_PAGE_NX; >> >> We will have a try in a minute, and report back later. > And it works，we can boot/reboot the system successfully, thank > you all the quick response and debug! I think I'll just submit the attached patch if there are no objections (and if it works, of course!). I just stuck the NX clearing at the bottom.
From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> 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.hansen@xxxxxxxxxxxxxxx> Cc: Ning Sun <ning.sun@xxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> Cc: x86@xxxxxxxxxx Cc: tboot-devel@xxxxxxxxxxxxxxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx --- 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; } _
