Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by PUD_SIZE

Song Liu
2019-08-20T14:00:43Z

> On Aug 20, 2019, at 6:55 AM, Rik van Riel <riel@fb.com> wrote:
>
> On Tue, 2019-08-20 at 09:21 -0400, Song Liu wrote:
>>> On Aug 20, 2019, at 4:16 AM, Thomas Gleixner <tglx@linutronix.de>
>>> wrote:
>>>
>>> On Tue, 20 Aug 2019, Peter Zijlstra wrote:
>>>> What that code wants to do is skip to the end of the pud, a
>>>> pmd_size
>>>> increase will not do that. And right below this, there's a second
>>>> instance of this exact pattern.
>>>>
>>>> Did I get the below right?
>>>>
>>>> ---
>>>> diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
>>>> index b196524759ec..32b20b3cb227 100644
>>>> --- a/arch/x86/mm/pti.c
>>>> +++ b/arch/x86/mm/pti.c
>>>> @@ -330,12 +330,14 @@ pti_clone_pgtable(unsigned long start,
>>>> unsigned long end,
>>>>
>>>> pud = pud_offset(p4d, addr);
>>>> if (pud_none(*pud)) {
>>>> + addr &= PUD_MASK;
>>>> addr += PUD_SIZE;
>>>
>>> round_up(addr, PUD_SIZE);
>>
>> I guess we need "round_up(addr + PMD_SIZE, PUD_SIZE)".
>
> What does that do if start is less than PMD_SIZE
> away from the next PUD_SIZE boundary?

Great point!

>
> How about: round_up(addr + 1, PUD_SIZE) ?

Yes. How about this?

=========================== 8< ============================

From 9ae74cff4faf4710a11cb8da4c4a3f3404bd9fdd Mon Sep 17 00:00:00 2001
From: Song Liu <songliubraving@fb.com>
Date: Mon, 19 Aug 2019 23:59:47 -0700
Subject: [PATCH] x86/mm/pti: in pti_clone_pgtable(), increase addr properly

Before 32-bit support, pti_clone_pmds() always adds PMD_SIZE to addr.
This behavior changes after the 32-bit support: pti_clone_pgtable()
increases addr by PUD_SIZE for pud_none(*pud) case, and increases addr by
PMD_SIZE for pmd_none(*pmd) case. However, this is not accurate because
addr may not be PUD_SIZE/PMD_SIZE aligned.

Fix this issue by properly rounding up addr to next PUD_SIZE/PMD_SIZE
in these two cases.

Cc: stable@vger.kernel.org # v4.19+
Fixes: 16a3fe634f6a ("x86/mm/pti: Clone kernel-image on PTE level for 32 bit")
Signed-off-by: Song Liu <songliubraving@fb.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
---
arch/x86/mm/pti.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index b196524759ec..1337494e22ef 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -330,13 +330,13 @@ pti_clone_pgtable(unsigned long start, unsigned long end,

pud = pud_offset(p4d, addr);
if (pud_none(*pud)) {
- addr += PUD_SIZE;
+ addr = round_up(addr + 1, PUD_SIZE);
continue;
}

pmd = pmd_offset(pud, addr);
if (pmd_none(*pmd)) {
- addr += PMD_SIZE;
+ addr = round_up(addr + 1, PMD_SIZE);
continue;
}

--
2.17.1