keys: Fix linking a duplicate key to a keyring's assoc_array

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

keys: Fix linking a duplicate key to a keyring's assoc_array

When making a DNS query inside the kernel using dns_query(), the request
code can in rare cases end up creating a duplicate index key in the
assoc_array of the destination keyring. It is eventually found by
a BUG_ON() check in the assoc_array implementation and results in
a crash.

Example report:
[2158499.700025] kernel BUG at ../lib/assoc_array.c:652!
[2158499.700039] invalid opcode: 0000 [#1] SMP PTI
[2158499.700065] CPU: 3 PID: 31985 Comm: kworker/3:1 Kdump: loaded Not tainted 5.3.18-150300.59.90-default #1 SLE15-SP3
[2158499.700096] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 11/12/2020
[2158499.700351] Workqueue: cifsiod cifs_resolve_server [cifs]
[2158499.700380] RIP: 0010:assoc_array_insert+0x85f/0xa40
[2158499.700401] Code: ff 74 2b 48 8b 3b 49 8b 45 18 4c 89 e6 48 83 e7 fe e8 95 ec 74 00 3b 45 88 7d db 85 c0 79 d4 0f 0b 0f 0b 0f 0b e8 41 f2 be ff <0f> 0b 0f 0b 81 7d 88 ff ff ff 7f 4c 89 eb 4c 8b ad 58 ff ff ff 0f
[2158499.700448] RSP: 0018:ffffc0bd6187faf0 EFLAGS: 00010282
[2158499.700470] RAX: ffff9f1ea7da2fe8 RBX: ffff9f1ea7da2fc1 RCX: 0000000000000005
[2158499.700492] RDX: 0000000000000000 RSI: 0000000000000005 RDI: 0000000000000000
[2158499.700515] RBP: ffffc0bd6187fbb0 R08: ffff9f185faf1100 R09: 0000000000000000
[2158499.700538] R10: ffff9f1ea7da2cc0 R11: 000000005ed8cec8 R12: ffffc0bd6187fc28
[2158499.700561] R13: ffff9f15feb8d000 R14: ffff9f1ea7da2fc0 R15: ffff9f168dc0d740
[2158499.700585] FS: 0000000000000000(0000) GS:ffff9f185fac0000(0000) knlGS:0000000000000000
[2158499.700610] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[2158499.700630] CR2: 00007fdd94fca238 CR3: 0000000809d8c006 CR4: 00000000003706e0
[2158499.700702] Call Trace:
[2158499.700741] ? key_alloc+0x447/0x4b0
[2158499.700768] ? __key_link_begin+0x43/0xa0
[2158499.700790] __key_link_begin+0x43/0xa0
[2158499.700814] request_key_and_link+0x2c7/0x730
[2158499.700847] ? dns_resolver_read+0x20/0x20 [dns_resolver]
[2158499.700873] ? key_default_cmp+0x20/0x20
[2158499.700898] request_key_tag+0x43/0xa0
[2158499.700926] dns_query+0x114/0x2ca [dns_resolver]
[2158499.701127] dns_resolve_server_name_to_ip+0x194/0x310 [cifs]
[2158499.701164] ? scnprintf+0x49/0x90
[2158499.701190] ? __switch_to_asm+0x40/0x70
[2158499.701211] ? __switch_to_asm+0x34/0x70
[2158499.701405] reconn_set_ipaddr_from_hostname+0x81/0x2a0 [cifs]
[2158499.701603] cifs_resolve_server+0x4b/0xd0 [cifs]
[2158499.701632] process_one_work+0x1f8/0x3e0
[2158499.701658] worker_thread+0x2d/0x3f0
[2158499.701682] ? process_one_work+0x3e0/0x3e0
[2158499.701703] kthread+0x10d/0x130
[2158499.701723] ? kthread_park+0xb0/0xb0
[2158499.701746] ret_from_fork+0x1f/0x40

The situation occurs as follows:
* Some kernel facility invokes dns_query() to resolve a hostname, for
example, "abcdef". The function registers its global DNS resolver
cache as current->cred.thread_keyring and passes the query to
request_key_net() -> request_key_tag() -> request_key_and_link().
* Function request_key_and_link() creates a keyring_search_context
object. Its match_data.cmp method gets set via a call to
type->match_preparse() (resolves to dns_resolver_match_preparse()) to
dns_resolver_cmp().
* Function request_key_and_link() continues and invokes
search_process_keyrings_rcu() which returns that a given key was not
found. The control is then passed to request_key_and_link() ->
construct_alloc_key().
* Concurrently to that, a second task similarly makes a DNS query for
"abcdef." and its result gets inserted into the DNS resolver cache.
* Back on the first task, function construct_alloc_key() first runs
__key_link_begin() to determine an assoc_array_edit operation to
insert a new key. Index keys in the array are compared exactly as-is,
using keyring_compare_object(). The operation
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