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

Dave Hansen
2018-01-06T07:55:40Z

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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.

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From: Dave Hansen <dave.hansen@linux.intel.com>

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@linux.intel.com>
Cc: Ning Sun <ning.sun@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: tboot-devel@lists.sourceforge.net
Cc: linux-kernel@vger.kernel.org
---

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;
}

_