Re: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do_scan()

From: Chris Wright
Date: Thu Jun 02 2011 - 12:49:26 EST

* Andrea Righi (andrea@xxxxxxxxxxxxxxx) wrote:
> mmh.. I can reproduce the bug also with the standard ubuntu (11.04)
> kernel. Could you post your .config?

Andrea (Righi), can you tell me if this WARN fires? This looks
like a pure race between removing from list and checking list, i.e.
insufficient locking.

ksm_scan.mm_slot == the only registered mm

CPU 1 (bug program) CPU 2 (ksmd)
list_empty() is false
slot == &ksm_mm_head (but list is now empty_)

diff --git a/mm/ksm.c b/mm/ksm.c
index 942dfc7..ab79a92 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1301,6 +1301,7 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
slot = list_entry(slot->, struct mm_slot, mm_list);
ksm_scan.mm_slot = slot;
+ WARN_ON(slot == &ksm_mm_head);
ksm_scan.address = 0;
ksm_scan.rmap_list = &slot->rmap_list;
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