| From | Paul Gortmaker <> | Subject | [34-longterm 076/179] ksm: fix NULL pointer dereference in scan_get_next_rmap_item() | Date | Mon, 14 May 2012 22:12:52 -0400 |
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From: Hugh Dickins <hughd@google.com>
------------------- This is a commit scheduled for the next v2.6.34 longterm release. http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git If you see a problem with using this for longterm, please comment. -------------------
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: Paul Gortmaker <paul.gortmaker@windriver.com> --- mm/ksm.c | 6 ++++++ 1 file changed, 6 insertions(+)
diff --git a/mm/ksm.c b/mm/ksm.c index 956880f..5e8e222 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -1270,6 +1270,12 @@ 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; -- 1.7.9.6
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