Commit message: mlock: fix mlock count can not decrease in race condition

Kefeng reported that when running the follow test, the mlock count in
meminfo will increase permanently:

 [1] testcase
 linux:~ # cat test_mlockal
 grep Mlocked /proc/meminfo
  for j in `seq 0 10`
  do
 	for i in `seq 4 15`
 	do
 		./p_mlockall >> log &
 	done
 	sleep 0.2
 done
 # wait some time to let mlock counter decrease and 5s may not enough
 sleep 5
 grep Mlocked /proc/meminfo

 linux:~ # cat p_mlockall.c
 #include <sys/mman.h>
 #include <stdlib.h>
 #include <stdio.h>

 #define SPACE_LEN	4096

 int main(int argc, char ** argv)
 {
	 	int ret;
	 	void *adr = malloc(SPACE_LEN);
	 	if (!adr)
	 		return -1;

	 	ret = mlockall(MCL_CURRENT | MCL_FUTURE);
	 	printf("mlcokall ret = %d\n", ret);

	 	ret = munlockall();
	 	printf("munlcokall ret = %d\n", ret);

	 	free(adr);
	 	return 0;
	 }

In __munlock_pagevec() we should decrement NR_MLOCK for each page where
we clear the PageMlocked flag.  Commit 1ebb7cc6a583 ("mm: munlock: batch
NR_MLOCK zone state updates") has introduced a bug where we don't
decrement NR_MLOCK for pages where we clear the flag, but fail to
isolate them from the lru list (e.g.  when the pages are on some other
cpu's percpu pagevec).  Since PageMlocked stays cleared, the NR_MLOCK
accounting gets permanently disrupted by this.

Fix it by counting the number of page whose PageMlock flag is cleared.

Fixes: 1ebb7cc6a583 (" mm: munlock: batch NR_MLOCK zone state updates")
Link: http://lkml.kernel.org/r/1495678405-54569-1-git-send-email-xieyisheng1@huawei.com
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Reported-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Tested-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Joern Engel <joern@logfs.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michel Lespinasse <walken@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Cc: zhongjiang <zhongjiang@huawei.com>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2017-18221
CWE: CWE-20
Vulnerability description: The __munlock_pagevec function in mm/mlock.c in the Linux kernel before 4.11.4 allows local users to cause a denial of service (NR_MLOCK accounting corruption) via crafted use of mlockall and munlockall system calls.

Project: linux
File: mlock.c

static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
 {
 	int i;
 	int nr = pagevec_count(pvec);
	int delta_munlocked;
 	struct pagevec pvec_putback;
 	int pgrescued = 0;
 
	pagevec_init(&pvec_putback, 0);

	/* Phase 1: page isolation */
	spin_lock_irq(zone_lru_lock(zone));
	for (i = 0; i < nr; i++) {
		struct page *page = pvec->pages[i];

		if (TestClearPageMlocked(page)) {
			/*
			 * We already have pin from follow_page_mask()
			 * so we can spare the get_page() here.
			 */
			if (__munlock_isolate_lru_page(page, false))
 				continue;
 			else
 				__munlock_isolation_failed(page);
 		}
 
 		/*
		 * We won't be munlocking this page in the next phase
		 * but we still need to release the follow_page_mask()
		 * pin. We cannot do it under lru_lock however. If it's
		 * the last pin, __page_cache_release() would deadlock.
		 */
 		pagevec_add(&pvec_putback, pvec->pages[i]);
 		pvec->pages[i] = NULL;
 	}
	delta_munlocked = -nr + pagevec_count(&pvec_putback);
 	__mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);
 	spin_unlock_irq(zone_lru_lock(zone));
 
	/* Now we can release pins of pages that we are not munlocking */
	pagevec_release(&pvec_putback);

	/* Phase 2: page munlock */
	for (i = 0; i < nr; i++) {
		struct page *page = pvec->pages[i];

		if (page) {
			lock_page(page);
			if (!__putback_lru_fast_prepare(page, &pvec_putback,
					&pgrescued)) {
				/*
				 * Slow path. We don't want to lose the last
				 * pin before unlock_page()
				 */
				get_page(page); /* for putback_lru_page() */
				__munlock_isolated_page(page);
				unlock_page(page);
				put_page(page); /* from follow_page_mask() */
			}
		}
	}

	/*
	 * Phase 3: page putback for pages that qualified for the fast path
	 * This will also call put_page() to return pin from follow_page_mask()
	 */
	if (pagevec_count(&pvec_putback))
		__putback_lru_fast(&pvec_putback, pgrescued);
}