bcache: fix NULL pointer in cache_set_flush()

In the Linux kernel, the following vulnerability has been resolved:

bcache: fix NULL pointer in cache_set_flush()

1. LINE#1794 - LINE#1887 is some codes about function of
bch_cache_set_alloc().
2. LINE#2078 - LINE#2142 is some codes about function of
register_cache_set().
3. register_cache_set() will call bch_cache_set_alloc() in LINE#2098.

1794 struct cache_set *bch_cache_set_alloc(struct cache_sb *sb)
1795 {
...
1860 if (!(c->devices = kcalloc(c->nr_uuids, sizeof(void *), GFP_KERNEL)) ||
1861 mempool_init_slab_pool(&c->search, 32, bch_search_cache) ||
1862 mempool_init_kmalloc_pool(&c->bio_meta, 2,
1863 sizeof(struct bbio) + sizeof(struct bio_vec) *
1864 bucket_pages(c)) ||
1865 mempool_init_kmalloc_pool(&c->fill_iter, 1, iter_size) ||
1866 bioset_init(&c->bio_split, 4, offsetof(struct bbio, bio),
1867 BIOSET_NEED_BVECS|BIOSET_NEED_RESCUER) ||
1868 !(c->uuids = alloc_bucket_pages(GFP_KERNEL, c)) ||
1869 !(c->moving_gc_wq = alloc_workqueue("bcache_gc",
1870 WQ_MEM_RECLAIM, 0)) ||
1871 bch_journal_alloc(c) ||
1872 bch_btree_cache_alloc(c) ||
1873 bch_open_buckets_alloc(c) ||
1874 bch_bset_sort_state_init(&c->sort, ilog2(c->btree_pages)))
1875 goto err;
^^^^^^^^
1876
...
1883 return c;
1884 err:
1885 bch_cache_set_unregister(c);
^^^^^^^^^^^^^^^^^^^^^^^^^^^
1886 return NULL;
1887 }
...
2078 static const char *register_cache_set(struct cache *ca)
2079 {
...
2098 c = bch_cache_set_alloc(&ca->sb);
2099 if (!c)
2100 return err;
^^^^^^^^^^
...
2128 ca->set = c;
2129 ca->set->cache[ca->sb.nr_this_dev] = ca;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
2138 return NULL;
2139 err:
2140 bch_cache_set_unregister(c);
2141 return err;
2142 }

(1) If LINE#1860 - LINE#1874 is true, then do 'goto err'(LINE#1875) and
call bch_cache_set_unregister()(LINE#1885).
(2) As (1) return NULL(LINE#1886), LINE#2098 - LINE#2100 would return.
(3) As (2) has returned, LINE#2128 - LINE#2129 would do *not* give the
value to c->cache[], it means that c->cache[] is NULL.

LINE#1624 - LINE#1665 is some codes about function of cache_set_flush().
As (1), in LINE#1885 call
bch_cache_set_unregister()
---> bch_cache_set_stop()
---> closure_queue()
-.-> cache_set_flush() (as below LINE#1624)

1624 static void cache_set_flush(struct closure *cl)
1625 {
...
1654 for_each_cache(ca, c, i)
1655 if (ca->alloc_thread)
^^
1656 kthread_stop(ca->alloc_thread);
...
1665 }

(4) In LINE#1655 ca is NULL(see (3)) in cache_set_flush() then the
kernel crash occurred as below:
[ 846.712887] bcache: register_cache() error drbd6: cannot allocate memory
[ 846.713242] bcache: register_bcache() error : failed to register device
[ 846.713336] bcache: cache_set_free() Cache set 2f84bdc1-498a-4f2f-98a7-01946bf54287 unregistered
[ 846.713768] BUG: unable to handle kernel NULL pointer dereference at 00000000000009f8
[ 846.714790] PGD 0 P4D 0
[ 846.715129] Oops: 0000 [#1] SMP PTI
[ 846.715472] CPU: 19 PID: 5057 Comm: kworker/19:16 Kdump: loaded Tainted: G OE --------- - - 4.18.0-147.5.1.el8_1.5es.3.x86_64 #1
[ 846.716082] Hardware name: ESPAN GI-25212/X11DPL-i, BIOS 2.1 06/15/2018
[ 846.716451] Workqueue: events cache_set_flush [bcache]
[ 846.716808] RIP: 0010:cache_set_flush+0xc9/0x1b0 [bcache]
[ 846.717155] Code: 00 4c 89 a5 b0 03 00 00 48 8b 85 68 f6 ff ff a8 08 0f 84 88 00 00 00 31 db 66 83 bd 3c f7 ff ff 00 48 8b 85 48 ff ff ff 74 28 <48> 8b b8 f8 09 00 0
---truncated---