Patchwork [Lucid,CVE-2013-0309,1/1] mm: thp: fix pmd_present for split_huge_page and PROT_NONE with THP

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Submitter Luis Henriques
Date Feb. 21, 2013, 12:02 p.m.
Message ID <>
Download mbox | patch
Permalink /patch/222269/
State New
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Luis Henriques - Feb. 21, 2013, 12:02 p.m.
From: Andrea Arcangeli <>



In many places !pmd_present has been converted to pmd_none.  For pmds
that's equivalent and pmd_none is quicker so using pmd_none is better.

However (unless we delete pmd_present) we should provide an accurate
pmd_present too.  This will avoid the risk of code thinking the pmd is non
present because it's under __split_huge_page_map, see the pmd_mknotpresent
there and the comment above it.

If the page has been mprotected as PROT_NONE, it would also lead to a
pmd_present false negative in the same way as the race with

Because the PSE bit stays on at all times (both during split_huge_page and
when the _PAGE_PROTNONE bit get set), we could only check for the PSE bit,
but checking the PROTNONE bit too is still good to remember pmd_present
must always keep PROT_NONE into account.

This explains a not reproducible BUG_ON that was seldom reported on the

The same issue is in pmd_large, it would go wrong with both PROT_NONE and
if it races with split_huge_page.

Signed-off-by: Andrea Arcangeli <>
Acked-by: Rik van Riel <>
Cc: Johannes Weiner <>
Cc: Hugh Dickins <>
Cc: Mel Gorman <>
Cc: <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
(cherry picked from commit 027ef6c87853b0a9df53175063028edb4950d476)

Signed-off-by: Luis Henriques <>
 arch/x86/include/asm/pgtable.h | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)
Tim Gardner - Feb. 21, 2013, 1 p.m.


diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index af6fd36..b6e3fd6 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -134,8 +134,7 @@  static inline unsigned long pmd_pfn(pmd_t pmd)
 static inline int pmd_large(pmd_t pte)
-	return (pmd_flags(pte) & (_PAGE_PSE | _PAGE_PRESENT)) ==
+	return pmd_flags(pte) & _PAGE_PSE;
 static inline pte_t pte_set_flags(pte_t pte, pteval_t set)
@@ -327,7 +326,13 @@  static inline int pte_hidden(pte_t pte)
 static inline int pmd_present(pmd_t pmd)
-	return pmd_flags(pmd) & _PAGE_PRESENT;
+	/*
+	 * Checking for _PAGE_PSE is needed too because
+	 * split_huge_page will temporarily clear the present bit (but
+	 * the _PAGE_PSE flag will remain set at all times while the
+	 * _PAGE_PRESENT bit is clear).
+	 */
+	return pmd_flags(pmd) & (_PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PSE);
 static inline int pmd_none(pmd_t pmd)