[PATCH] mm: fix NULL pointer dereference in wp_page_reuse()

[Qi Zheng <zhengqi.arch@bytedance.com>]

The vmf->page can be NULL when the wp_page_reuse() is invoked
by wp_pfn_shared(), it will cause the following panic:

[   12.981200] BUG: kernel NULL pointer dereference, address: 000000000000008
[   12.981954] #PF: supervisor read access in kernel mode
[   12.982615] #PF: error_code(0x0000) - not-present page
[   12.983146] PGD 0 P4D 0
[   12.983417] Oops: 0000 [#1] PREEMPT SMP PTI
[   12.983846] CPU: 18 PID: 923 Comm: Xorg Not tainted 5.19.0-rc8.bm.1-amd64 #263
[   12.984606] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g14
[   12.985760] RIP: 0010:_compound_head+0x0/0x40
[...]
[   12.995932] Call Trace:
[   12.996199]  <TASK>
[   12.996424]  wp_page_reuse+0x1c/0xa0
[   12.996800]  do_wp_page+0x1a5/0x3f0
[   12.997169]  __handle_mm_fault+0x8cf/0xd20
[   12.997587]  ? _raw_spin_unlock_irqrestore+0x23/0x35
[   12.998105]  handle_mm_fault+0xd5/0x2a0
[   12.998499]  do_user_addr_fault+0x1d0/0x680
[   12.998930]  exc_page_fault+0x78/0x170
[   12.999321]  asm_exc_page_fault+0x22/0x30

To fix it, this patch performs a NULL pointer check before
dereferencing the vmf->page.

Fixes: 6c287605fd56 ("mm: remember exclusively mapped anonymous pages with PG_anon_exclusive")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
 mm/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 4cf7d4b6c950..9174918ce3f7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3043,7 +3043,7 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
 	pte_t entry;
 
 	VM_BUG_ON(!(vmf->flags & FAULT_FLAG_WRITE));
-	VM_BUG_ON(PageAnon(page) && !PageAnonExclusive(page));
+	VM_BUG_ON(page && PageAnon(page) && !PageAnonExclusive(page));
 
 	/*
 	 * Clear the pages cpupid information as the existing
-- 
2.20.1