[PATCH -next] mm/memory: fix null pointer dereference in fault_dirty_shared_page

[Yuntao Liu <liuyuntao12@huawei.com>]

Page mapping with "VM_READ|VM_WRITE|VM_MAYREAD|VM_MAYWRITE|VM_SHARED",
the first time accessing this address through a write operation will
trigger a do_shared_fault, if mapping is anonymous, it can lead to a
null pointer dereference.

[   23.232336][  T195] Call trace:
[   23.232542][  T195]  file_update_time+0x2c/0xd8
[   23.232801][  T195]  fault_dirty_shared_page+0x1a0/0x220
[   23.233099][  T195]  do_shared_fault+0xe8/0x240
[   23.233374][  T195]  do_fault+0x78/0x240
[   23.233629][  T195]  handle_pte_fault+0x1f0/0x3f0
[   23.233905][  T195]  __handle_mm_fault+0x2b0/0x548
[   23.234186][  T195]  handle_mm_fault+0xd4/0x2f8
[   23.234462][  T195]  do_page_fault+0x2f0/0x5f8
[   23.234727][  T195]  do_translation_fault+0x8c/0xc8
[   23.235021][  T195]  do_mem_abort+0x68/0x100
[   23.235283][  T195]  el0_da+0x4c/0x1a8
[   23.235551][  T195]  el0t_64_sync_handler+0xe4/0x158
[   23.235861][  T195]  el0t_64_sync+0x37c/0x380

Signed-off-by: Yuntao Liu <liuyuntao12@huawei.com>
---
 mm/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index eaf98d518289..8106ef8a5036 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3412,7 +3412,7 @@ static vm_fault_t fault_dirty_shared_page(struct vm_fault *vmf)
 	mapping = folio_raw_mapping(folio);
 	folio_unlock(folio);
 
-	if (!page_mkwrite)
+	if (!page_mkwrite && vma->vm_file)
 		file_update_time(vma->vm_file);
 
 	/*
-- 
2.34.1