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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@localhost.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@sympatico.ca>
Signed-off-by: Joerg Roedel <jroedel@suse.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
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