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>
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Родитель
c7cbb02222
Коммит
2b472611a3
6
mm/ksm.c
6
mm/ksm.c
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@ -1302,6 +1302,12 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
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slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
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ksm_scan.mm_slot = slot;
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spin_unlock(&ksm_mmlist_lock);
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/*
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* Although we tested list_empty() above, a racing __ksm_exit
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* of the last mm on the list may have removed it since then.
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*/
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if (slot == &ksm_mm_head)
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return NULL;
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next_mm:
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ksm_scan.address = 0;
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ksm_scan.rmap_list = &slot->rmap_list;
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