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@xxxxxxxxxxxxx>
---
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
