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

[Dave Hansen]

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!).

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;
 }
 
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