From: Hugh Dickins Date: Wed, 15 Jun 2011 22:08:58 +0000 (-0700) Subject: ksm: fix NULL pointer dereference in scan_get_next_rmap_item() X-Git-Tag: v2.6.35.14~85 X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=a5c70c5bbd56baea711ddd0dfe3e617d0ba20e6f;p=karo-tx-linux.git ksm: fix NULL pointer dereference in scan_get_next_rmap_item() 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 #include #include #include #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 Tested-by: Andrea Righi Cc: Andrea Arcangeli Signed-off-by: Hugh Dickins Signed-off-by: Chris Wright Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman Signed-off-by: Andi Kleen --- diff --git a/mm/ksm.c b/mm/ksm.c index 79cd773faa85..2db786a576ac 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;