mm: fix null pointer dereference in vm_normal_page()

One usage of vm_normal_page() is:

    struct page *page = vm_normal_page(gate_vma, start, *pte);

where gate_vma is returned by get_gate_vma() which can be NULL. So let vm_normal_page return NULL when vma is NULL.

---

index baa999e..e428aa6 100644
@@ -493,6 +493,9 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
 {
 	unsigned long pfn = pte_pfn(pte);
 
+	if (!vma)
+		return NULL;
+
 	if (HAVE_PTE_SPECIAL) {
 		if (likely(!pte_special(pte)))
 			goto check_pfn;

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I assume you refer to __get_user_pages()...?

This function checks whether the address is in the gate area and only iff so requests the VMA representing it.

If you really did see an oops that is worked-around by your patch, then the in_gate_area()/get_gate_vma() in question are broken.