[Patch] mm: fix null pointer dereference in vm_normal_page()

[AmÃrico Wang]

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.

Signed-off-by: WANG Cong <wangcong@xxxxxxxxx>
Cc: Nick Piggin <nickpiggin@xxxxxxxxxxxx>

---
diff --git a/mm/memory.c b/mm/memory.c
index baa999e..e428aa6 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -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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