linux-kernel - [PATCH] x86/pgtable: Don't set huge pud/pmd on non-leaf entries

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Date:   Wed, 11 Apr 2018 17:24:38 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>, aliguori@...zon.com,
        daniel.gruss@...k.tugraz.at, hughd@...gle.com, keescook@...gle.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        jroedel@...e.de, "David H. Gutteridge" <dhgutteridge@...patico.ca>
Subject: [PATCH] x86/pgtable: Don't set huge pud/pmd on non-leaf entries

Hi Ingo, Thomas,

below patch is an update to this series and I plan to include it in the
next post. David H. Gutteridge (CC'ed) was so kind to do additional
testing of these patches and found that a BUG_ON was triggered in
vmalloc_sync_one() when PTI is enabled.

My debugging showed that the bug was present before my patches but that
they uncovered this bug by setting SHARED_KERNEL_PMD to 0 when PTI is
enabled.

So please have a look at the issue description below and the patch and
let me know what you think. I know there are other and better variants
to fix that, but I felt a rework to make the generic ioremap code more
safe against this is out-of-scope at least for this patch-set.

Thanks,

	Joerg

>From 48cb37800794be8f0611e87f9ef23774c3b36ecc Mon Sep 17 00:00:00 2001
From: Joerg Roedel <joro@...alhost.localdomain>
Date: Wed, 11 Apr 2018 15:55:14 +0200
Subject: [PATCH] x86/pgtable: Don't set huge pud/pmd on non-leaf entries

The pmd_set_huge() and pud_set_huge() functions are used from
generic ioremap code to establish large mappings where this
is possible.

But the generic ioremap code does not check whether the
pmd/pud entries are already populated with a non-leaf entry,
so that any page-table pages these entries point to will be
lost.

Further, on x86-32 with SHARED_KERNEL_PMD=0, this causes a
BUG_ON() in vmalloc_sync_one() when pmd entries are synced
from swapper_pg_dir to the current page-table. This happens
because the pmd entry from swapper_pg_dir was promoted to a
huge-page entry while the current pgd still contains the
non-leaf entry. Because both entries are present and point
to a different page, the BUG_ON triggers.

This was actually triggered with pti-x32 enabled in a KVM
virtual machine by the graphics driver.

A real and better fix for that would be to improve the
page-table handling in the generic ioremap code. But that is
out-of-scope for this patch-set and left for later work.

Reported-by: David H. Gutteridge <dhgutteridge@...patico.ca>
Signed-off-by: Joerg Roedel <jroedel@...e.de>
---
 arch/x86/mm/pgtable.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index d95bc7b1ffb4..6b72d2805058 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/mm.h>
 #include <linux/gfp.h>
+#include <linux/hugetlb.h>
 #include <asm/pgalloc.h>
 #include <asm/pgtable.h>
 #include <asm/tlb.h>
@@ -699,6 +700,10 @@ int pud_set_huge(pud_t *pud, phys_addr_t addr, pgprot_t prot)
 	    (mtrr != MTRR_TYPE_WRBACK))
 		return 0;

+	/* Bail out if we are we on a populated non-leaf entry */
+	if (pud_present(*pud) && !pud_huge(*pud))
+		return 0;
+
 	prot = pgprot_4k_2_large(prot);

 	set_pte((pte_t *)pud, pfn_pte(
@@ -727,6 +732,10 @@ int pmd_set_huge(pmd_t *pmd, phys_addr_t addr, pgprot_t prot)
 		return 0;
 	}

+	/* Bail out if we are we on a populated non-leaf entry */
+	if (pmd_present(*pmd) && !pmd_huge(*pmd))
+		return 0;
+
 	prot = pgprot_4k_2_large(prot);

 	set_pte((pte_t *)pmd, pfn_pte(
-- 
2.13.6