Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page tables (core patch)

[Hanjun Guo]

On 2018/1/6 15:55, Dave Hansen wrote:
> 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 worksïwe 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!).

We tested that placing the NX clearing after pud_alloc(), and it works,
patch below should work as well.

> 
> I just stuck the NX clearing at the bottom.
> 
> 
> pti-tboot-fix.patch
> 
> 
> 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;
>  }

Thanks
Hanjun