ARM: 6955/1: cmpxchg syscall should data abort if page not write

If the page to cmpxchg is user mode read only (not write),
we should simulate a data abort first.
Signed-off-by: NPo-Yu Chuang <ratbert@faraday-tech.com>
Acked-by: NNicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/traps.c

@@ -563,7 +563,7 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
 		if (!pmd_present(*pmd))
 			goto bad_access;
 		pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
-		if (!pte_present(*pte) || !pte_dirty(*pte)) {
+		if (!pte_present(*pte) || !pte_write(*pte) || !pte_dirty(*pte)) {
 			pte_unmap_unlock(pte, ptl);
 			goto bad_access;
 		}