* Hugh Dickins (hughd@xxxxxxxxxx) wrote: > On Thu, 2 Jun 2011, Chris Wright wrote: > > Close this race by revalidating that the new slot is not simply the list > > head again. > > Remarkably similar to my patch: it must be good! Indeed ;) > But yours appears to be more popular - thanks, Chris. But I like the comment in yours better... Andrew, here's a refresh w/ the acks/tested-bys/etc + AndreaR's test case in the changelog. thanks, -chris -- Subject: [PATCH] ksm: fix NULL pointer dereference in scan_get_next_rmap_item From: Hugh Dickins <hughd@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx> Tested-by: Andrea Righi <andrea@xxxxxxxxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: stable@xxxxxxxxxx Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx> --- mm/ksm.c | 6 ++++++ 1 files changed, 6 insertions(+), 0 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index d708b3e..9a68b0c 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -1302,6 +1302,12 @@ 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; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>