linux-kernel - [PATCH] [65/99] ksm: fix NULL pointer dereference in scan_get_next_rmap

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Message-Id: <20110727214904.7A3D82403FF@tassilo.jf.intel.com>
Date:	Wed, 27 Jul 2011 14:49:04 -0700 (PDT)
From:	Andi Kleen <andi@...stfloor.org>
To:	hughd@...gle.com, andrea@...terlinux.com, aarcange@...hat.com,
	chrisw@...s-sol.org, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, gregkh@...e.de, ak@...ux.intel.com,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	tim.bird@...sony.com
Subject: [PATCH] [65/99] ksm: fix NULL pointer dereference in scan_get_next_rmap_item()

2.6.35-longterm review patch.  If anyone has any objections, please let me know.

------------------
From: Hugh Dickins <hughd@...gle.com>

commit 2b472611a32a72f4a118c069c2d62a1a3f087afd upstream.

Andrea Righi reported a case where an exiting task can race against
ksmd::scan_get_next_rmap_item (http://lkml.org/lkml/2011/6/1/742) easily
triggering a NULL pointer dereference in ksmd.

ksm_scan.mm_slot == &ksm_mm_head with only one registered mm

CPU 1 (__ksm_exit)		CPU 2 (scan_get_next_rmap_item)
 				list_empty() is false
lock				slot == &ksm_mm_head
list_del(slot->mm_list)
(list now empty)
unlock
				lock
				slot = list_entry(slot->mm_list.next)
				(list is empty, so slot is still ksm_mm_head)
				unlock
				slot->mm == NULL ... Oops

Close this race by revalidating that the new slot is not simply the list
head again.

Andrea's test case:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>

#define BUFSIZE getpagesize()

int main(int argc, char **argv)
{
	void *ptr;

	if (posix_memalign(&ptr, getpagesize(), BUFSIZE) < 0) {
		perror("posix_memalign");
		exit(1);
	}
	if (madvise(ptr, BUFSIZE, MADV_MERGEABLE) < 0) {
		perror("madvise");
		exit(1);
	}
	*(char *)NULL = 0;

	return 0;
}

Reported-by: Andrea Righi <andrea@...terlinux.com>
Tested-by: Andrea Righi <andrea@...terlinux.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Signed-off-by: Chris Wright <chrisw@...s-sol.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
Signed-off-by: Andi Kleen <ak@...ux.intel.com>

---
 mm/ksm.c |    6 ++++++
 1 file changed, 6 insertions(+)

Index: linux-2.6.35.y/mm/ksm.c
===================================================================
--- linux-2.6.35.y.orig/mm/ksm.c
+++ linux-2.6.35.y/mm/ksm.c
@@ -1272,6 +1272,12 @@ static struct rmap_item *scan_get_next_r
 		slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
 		ksm_scan.mm_slot = slot;
 		spin_unlock(&ksm_mmlist_lock);
+		/*
+		 * Although we tested list_empty() above, a racing __ksm_exit
+		 * of the last mm on the list may have removed it since then.
+		 */
+		if (slot == &ksm_mm_head)
+			return NULL;
 next_mm:
 		ksm_scan.address = 0;
 		ksm_scan.rmap_list = &slot->rmap_list;
--
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