On 01/05/2018 08:54 PM, Hanjun Guo wrote: > Do you mean NX bit will be brought back later? I'm asking this because > I tested this patch which it fixed the boot panic issue but the system > will hang when rebooting the system, because rebooting will also call efi > then panic as NS bit is set. Wow, you're running a lot of very lighly-used code paths! You actually found a similar but totally separate issue from what I gather. Thank you immensely for the quick testing and bug reports! Could you test the attached fix? For those playing along at home, I think this will end up being needed for 4.15 and probably all the backports. I want to see if it works before I submit it for real, though.
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 | 7 +++++++ 1 file changed, 7 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 22:01:51.393553325 -0800 @@ -124,6 +124,13 @@ static int map_tboot_page(unsigned long pte_t *pte; pgd = pgd_offset(&tboot_mm, vaddr); + /* + * 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; + p4d = p4d_alloc(&tboot_mm, pgd, vaddr); if (!p4d) return -1; _
