1 files changed, 13 insertions, 0 deletions
diff --git a/mm/mremap.c b/mm/mremap.c
index 3f23715d3c69..7395564daa6c 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -384,6 +384,19 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
 	if (!vma || vma->vm_start > addr)
 		return ERR_PTR(-EFAULT);
 
+	/*
+	 * !old_len is a special case where an attempt is made to 'duplicate'
+	 * a mapping.  This makes no sense for private mappings as it will
+	 * instead create a fresh/new mapping unrelated to the original.  This
+	 * is contrary to the basic idea of mremap which creates new mappings
+	 * based on the original.  There are no known use cases for this
+	 * behavior.  As a result, fail such attempts.
+	 */
+	if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) {
+		pr_warn_once("%s (%d): attempted to duplicate a private mapping with mremap.  This is not supported.\n", current->comm, current->pid);
+		return ERR_PTR(-EINVAL);
+	}
+
 	if (is_vm_hugetlb_page(vma))
 		return ERR_PTR(-EINVAL);