path: root/mm/ksm.c
diff options
authorHugh Dickins <hughd@google.com>2011-06-15 15:08:58 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2011-06-15 20:04:02 -0700
commit2b472611a32a72f4a118c069c2d62a1a3f087afd (patch)
tree166ec2dea398e7760c30077fb329cf7f2ab08c2b /mm/ksm.c
parentrtc: fix build warnings in defconfigs (diff)
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>
Diffstat (limited to '')
1 files changed, 6 insertions, 0 deletions
diff --git a/mm/ksm.c b/mm/ksm.c
index d708b3ef2260..9a68b0cf0a1c 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;
+ /*
+ * 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;
ksm_scan.address = 0;
ksm_scan.rmap_list = &slot->rmap_list;