[PATCH] ksm: fix NULL pointer dereference in scan_get_next_rmap_item() - CVE-2011-2183

[Paolo Pisati]

From: Hugh Dickins <hughd at google.com>

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:

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 at betterlinux.com>
Tested-by: Andrea Righi <andrea at betterlinux.com>
Cc: Andrea Arcangeli <aarcange at redhat.com>
Signed-off-by: Hugh Dickins <hughd at google.com>
Signed-off-by: Chris Wright <chrisw at sous-sol.org>
Cc: <stable at kernel.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

CVE-2011-2183

(cherry picked from commit 2b472611a32a72f4a118c069c2d62a1a3f087afd)

BugLink: http://bugs.launchpad.net/bugs/869227

Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
---
 mm/ksm.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 79cd773..2db786a 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1272,6 +1272,12 @@ 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);
+		/*
+		 * 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;
-- 
1.7.5.4