userswap: check read and write permissions for swap-out pages

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6CAIM

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

Check the VM_READ and VM_WRITE flags of vma->vm_flags to determine
whether the read and write permissions of the swap-out page VA are
consistent with those of the swap-out buffer VA. If they are
inconsistent, the swap operation will fail.
Signed-off-by: NZhangPeng <zhangpeng362@huawei.com>

mm/userswap.c

@@ -311,8 +311,8 @@ static unsigned long do_user_swap(struct mm_struct *mm,
 			goto out_recover;
 
 		ret = -EACCES;
-		if (pgprot_val(old_vma->vm_page_prot) !=
-		    pgprot_val(new_vma->vm_page_prot))
+		if (!(old_vma->vm_flags & VM_WRITE) &&
+		    (new_vma->vm_flags & VM_WRITE))
 			goto out_recover;
 
 		ret = -ENXIO;