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

[Joerg Roedel]

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@xxxxxxxxxxxx>
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 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