s390/mm: fix write access check in gup_huge_pmd()

commit ba385c0594e723d41790ecfb12c610e6f90c7785 upstream.

The check for the _SEGMENT_ENTRY_PROTECT bit in gup_huge_pmd() is the
wrong way around. It must not be set for write==1, and not be checked for
write==0. Fix this similar to how it was fixed for ptes long time ago in
commit 25591b070336 ("[S390] fix get_user_pages_fast").

One impact of this bug would be unnecessarily using the gup slow path for
write==0 on r/w mappings. A potentially more severe impact would be that
gup_huge_pmd() will succeed for write==1 on r/o mappings.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
[bwh: Backported to 3.2:
 - Segment flag names are different
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/s390/mm/gup.c b/arch/s390/mm/gup.c
index 4ccf9f5..65dcae7 100644 (file)
--- a/arch/s390/mm/gup.c
+++ b/arch/s390/mm/gup.c
@@ -51,13 +51,12 @@ static inline int gup_pte_range(pmd_t *pmdp, pmd_t pmd, unsigned long addr,
 static inline int gup_huge_pmd(pmd_t *pmdp, pmd_t pmd, unsigned long addr,
                unsigned long end, int write, struct page **pages, int *nr)
 {
-       unsigned long mask, result;
        struct page *head, *page, *tail;
+       unsigned long mask;
        int refs;
 
-       result = write ? 0 : _SEGMENT_ENTRY_RO;
-       mask = result | _SEGMENT_ENTRY_INV;
-       if ((pmd_val(pmd) & mask) != result)
+       mask = (write ? _SEGMENT_ENTRY_RO : 0) | _SEGMENT_ENTRY_INV;
+       if ((pmd_val(pmd) & mask) != 0)
                return 0;
        VM_BUG_ON(!pfn_valid(pmd_val(pmd) >> PAGE_SHIFT));