linux-kernel - Re: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do

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Message-ID: <20110602164841.GK23047@sequoia.sous-sol.org>
Date:	Thu, 2 Jun 2011 09:48:41 -0700
From:	Chris Wright <chrisw@...s-sol.org>
To:	Andrea Righi <andrea@...terlinux.com>
Cc:	CAI Qian <caiqian@...hat.com>, Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Chris Wright <chrisw@...s-sol.org>, Mel Gorman <mel@....ul.ie>,
	Izik Eidus <ieidus@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do_scan()

* Andrea Righi (andrea@...terlinux.com) 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
lock
ksm_scan.mm_slot
list_del
unlock
				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->mm_list.next, struct mm_slot, mm_list);
 		ksm_scan.mm_slot = slot;
 		spin_unlock(&ksm_mmlist_lock);
+		WARN_ON(slot == &ksm_mm_head);
 next_mm:
 		ksm_scan.address = 0;
 		ksm_scan.rmap_list = &slot->rmap_list;
--
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