assoc_array: don't call compare_object() on a node

Changes since V1: fixed the description and added KASan warning.

In assoc_array_insert_into_terminal_node(), we call the
compare_object() method on all non-empty slots, even when they're
not leaves, passing a pointer to an unexpected structure to
compare_object(). Currently it causes an out-of-bound read access
in keyring_compare_object detected by KASan (see below). The issue
is easily reproduced with keyutils testsuite.
Only call compare_object() when the slot is a leave.

KASan warning:
==================================================================
BUG: KASAN: slab-out-of-bounds in keyring_compare_object+0x213/0x240 at addr ffff880060a6f838
Read of size 8 by task keyctl/1655
=============================================================================
BUG kmalloc-192 (Not tainted): kasan: bad access detected
-----------------------------------------------------------------------------

Disabling lock debugging due to kernel taint
INFO: Allocated in assoc_array_insert+0xfd0/0x3a60 age=69 cpu=1 pid=1647
	___slab_alloc+0x563/0x5c0
	__slab_alloc+0x51/0x90
	kmem_cache_alloc_trace+0x263/0x300
	assoc_array_insert+0xfd0/0x3a60
	__key_link_begin+0xfc/0x270
	key_create_or_update+0x459/0xaf0
	SyS_add_key+0x1ba/0x350
	entry_SYSCALL_64_fastpath+0x12/0x76
INFO: Slab 0xffffea0001829b80 objects=16 used=8 fp=0xffff880060a6f550 flags=0x3fff8000004080
INFO: Object 0xffff880060a6f740 @offset=5952 fp=0xffff880060a6e5d1

Bytes b4 ffff880060a6f730: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Object ffff880060a6f740: d1 e5 a6 60 00 88 ff ff 0e 00 00 00 00 00 00 00  ...`............
Object ffff880060a6f750: 02 cf 8e 60 00 88 ff ff 02 c0 8e 60 00 88 ff ff  ...`.......`....
Object ffff880060a6f760: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Object ffff880060a6f770: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Object ffff880060a6f780: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Object ffff880060a6f790: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Object ffff880060a6f7a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Object ffff880060a6f7b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Object ffff880060a6f7c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Object ffff880060a6f7d0: 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Object ffff880060a6f7e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Object ffff880060a6f7f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
CPU: 0 PID: 1655 Comm: keyctl Tainted: G    B           4.5.0-rc4-kasan+ #291
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
 0000000000000000 000000001b2800b4 ffff880060a179e0 ffffffff81b60491
 ffff88006c802900 ffff880060a6f740 ffff880060a17a10 ffffffff815e2969
 ffff88006c802900 ffffea0001829b80 ffff880060a6f740 ffff880060a6e650
Call Trace:
 [<ffffffff81b60491>] dump_stack+0x85/0xc4
 [<ffffffff815e2969>] print_trailer+0xf9/0x150
 [<ffffffff815e9454>] object_err+0x34/0x40
 [<ffffffff815ebe50>] kasan_report_error+0x230/0x550
 [<ffffffff819949be>] ? keyring_get_key_chunk+0x13e/0x210
 [<ffffffff815ec62d>] __asan_report_load_n_noabort+0x5d/0x70
 [<ffffffff81994cc3>] ? keyring_compare_object+0x213/0x240
 [<ffffffff81994cc3>] keyring_compare_object+0x213/0x240
 [<ffffffff81bc238c>] assoc_array_insert+0x86c/0x3a60
 [<ffffffff81bc1b20>] ? assoc_array_cancel_edit+0x70/0x70
 [<ffffffff8199797d>] ? __key_link_begin+0x20d/0x270
 [<ffffffff8199786c>] __key_link_begin+0xfc/0x270
 [<ffffffff81993389>] key_create_or_update+0x459/0xaf0
 [<ffffffff8128ce0d>] ? trace_hardirqs_on+0xd/0x10
 [<ffffffff81992f30>] ? key_type_lookup+0xc0/0xc0
 [<ffffffff8199e19d>] ? lookup_user_key+0x13d/0xcd0
 [<ffffffff81534763>] ? memdup_user+0x53/0x80
 [<ffffffff819983ea>] SyS_add_key+0x1ba/0x350
 [<ffffffff81998230>] ? key_get_type_from_user.constprop.6+0xa0/0xa0
 [<ffffffff828bcf4e>] ? retint_user+0x18/0x23
 [<ffffffff8128cc7e>] ? trace_hardirqs_on_caller+0x3fe/0x580
 [<ffffffff81004017>] ? trace_hardirqs_on_thunk+0x17/0x19
 [<ffffffff828bc432>] entry_SYSCALL_64_fastpath+0x12/0x76
Memory state around the buggy address:
 ffff880060a6f700: fc fc fc fc fc fc fc fc 00 00 00 00 00 00 00 00
 ffff880060a6f780: 00 00 00 00 00 00 00 00 00 00 00 fc fc fc fc fc
>ffff880060a6f800: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                                        ^
 ffff880060a6f880: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff880060a6f900: fc fc fc fc fc fc 00 00 00 00 00 00 00 00 00 00
==================================================================

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: stable@vger.kernel.org
