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authorHugh Dickins <hughd@google.com>2011-06-15 15:08:58 -0700
committerGreg Kroah-Hartman <gregkh@suse.de>2011-07-13 05:29:23 +0200
commit4553fbdf55fab298a213812d21988670a4bd0edd (patch)
treec06279bde95d4b8b71c04702f02154117ba0d6bf /mm/ksm.c
parentc49ed7e7d1461ed0ce88548de0d67c88784a99f3 (diff)
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 <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> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'mm/ksm.c')
-rw-r--r--mm/ksm.c6
1 files changed, 6 insertions, 0 deletions
diff --git a/mm/ksm.c b/mm/ksm.c
index e9501f833374..318ab79ab16a 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1215,6 +1215,12 @@ 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_item = list_entry(&slot->rmap_list,