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

[Stefan Bader]

On 14.10.2011 16:48, Paolo Pisati wrote:
> 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;