x86/pgtable: Don't set huge PUD/PMD on non-leaf entries

The pmd_set_huge() and pud_set_huge() functions are used from
the 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 <[email protected]>
Signed-off-by: Joerg Roedel <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Laight <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Eduardo Valentin <[email protected]>
Cc: Greg KH <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Waiman Long <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: joergroedel

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>
@@ -639,6 +640,10 @@ 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(
@@ -667,6 +672,10 @@ 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(