linux-kernel - Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by PUD

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Message-ID: <9A7CA4D3-76FB-479B-AC7A-FC3FD03B24DF@fb.com>
Date:   Tue, 20 Aug 2019 14:00:28 +0000
From:   Song Liu <songliubraving@...com>
To:     Rik van Riel <riel@...com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Joerg Roedel <jroedel@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by
 PUD_SIZE



> On Aug 20, 2019, at 6:55 AM, Rik van Riel <riel@...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@...utronix.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@...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@...r.kernel.org # v4.19+
Fixes: 16a3fe634f6a ("x86/mm/pti: Clone kernel-image on PTE level for 32 bit")
Signed-off-by: Song Liu <songliubraving@...com>
Cc: Joerg Roedel <jroedel@...e.de>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.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