The patch titled
ksm: fix NULL pointer dereference in scan_get_next_rmap_item()
has been added to the -mm tree. Its filename is
ksm-fix-null-pointer-dereference-in-scan_get_next_rmap_item.patch
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*** Remember to use Documentation/SubmitChecklist when testing your code ***
See http://userweb.kernel.org/~akpm/stuff/added-to-mm.txt to find
out what to do about this
The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
------------------------------------------------------
Subject: 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>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/ksm.c | 6 ++++++
1 file changed, 6 insertions(+)
diff -puN mm/ksm.c~ksm-fix-null-pointer-dereference-in-scan_get_next_rmap_item mm/ksm.c
--- a/mm/ksm.c~ksm-fix-null-pointer-dereference-in-scan_get_next_rmap_item
+++ a/mm/ksm.c
@@ -1302,6 +1302,12 @@ static struct rmap_item *scan_get_next_r
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;
_
Patches currently in -mm which might be from hughd@xxxxxxxxxx are
mm-fix-wrong-kunmap_atomic-pointer.patch
mm-migratec-dont-account-swapcache-as-shmem.patch
linux-next.patch
ksm-fix-null-pointer-dereference-in-scan_get_next_rmap_item.patch
mm-move-vmtruncate_range-to-truncatec.patch
mm-move-shmem-prototypes-to-shmem_fsh.patch
tmpfs-take-control-of-its-truncate_range.patch
tmpfs-add-shmem_read_mapping_page_gfp.patch
drm-ttm-use-shmem_read_mapping_page.patch
drm-i915-use-shmem_read_mapping_page.patch
drm-i915-use-shmem_truncate_range.patch
drm-i915-more-struct_mutex-locking.patch
mm-cleanup-descriptions-of-filler-arg.patch
mm-truncate-functions-are-in-truncatec.patch
mm-tidy-vmtruncate_range-and-related-functions.patch
mm-consistent-truncate-and-invalidate-loops.patch
mm-pincer-in-truncate_inode_pages_range.patch
tmpfs-no-need-to-use-i_lock.patch
prio_tree-debugging-patch.patch
--
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