[PATCH 3.16 016/204] s390/mm: fix write access check in gup_huge_pmd()

[Ben Hutchings]

3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/s390/mm/gup.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

--- a/arch/s390/mm/gup.c
+++ b/arch/s390/mm/gup.c
@@ -51,13 +51,12 @@ static inline int gup_pte_range(pmd_t *p
 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_PROTECT;
-	mask = result | _SEGMENT_ENTRY_INVALID;
-	if ((pmd_val(pmd) & mask) != result)
+	mask = (write ? _SEGMENT_ENTRY_PROTECT : 0) | _SEGMENT_ENTRY_INVALID;
+	if ((pmd_val(pmd) & mask) != 0)
 		return 0;
 	VM_BUG_ON(!pfn_valid(pmd_val(pmd) >> PAGE_SHIFT));