CVE-2011-2183: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition') (CWE-362)
value: Race condition in the scan_get_next_rmap_item function in mm/ksm.c in the Linux kernel before 2.6.39.3, when Kernel SamePage Merging (KSM) is enabled, allows local users to cause a denial of service (NULL pointer dereference) or possibly have unspecified other impact via a crafted application.
CWE description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
Published: 2012-06-13T10:24Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: mm/ksm.c
Commit date: 2011-06-15 20:04:02 -0700
Commit message: ksm: fix NULL pointer dereference in scan_get_next_rmap_item()

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

Diff: diff --git a/mm/ksm.c b/mm/ksm.c
index d708b3ef2260..9a68b0cf0a1c 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1300,10 +1300,16 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
 
 		spin_lock(&ksm_mmlist_lock);
 		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;
 	}
 

Fixed code: 		/*
		 * 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;