lib/alloc_tag: do not acquire non-existent lock in alloc_tag_top_users()

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

lib/alloc_tag: do not acquire non-existent lock in alloc_tag_top_users()

alloc_tag_top_users() attempts to lock alloc_tag_cttype->mod_lock even
when the alloc_tag_cttype is not allocated because:

1) alloc tagging is disabled because mem profiling is disabled
(!alloc_tag_cttype)
2) alloc tagging is enabled, but not yet initialized (!alloc_tag_cttype)
3) alloc tagging is enabled, but failed initialization
(!alloc_tag_cttype or IS_ERR(alloc_tag_cttype))

In all cases, alloc_tag_cttype is not allocated, and therefore
alloc_tag_top_users() should not attempt to acquire the semaphore.

This leads to a crash on memory allocation failure by attempting to
acquire a non-existent semaphore:

Oops: general protection fault, probably for non-canonical address 0xdffffc000000001b: 0000 [#3] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x00000000000000d8-0x00000000000000df]
CPU: 2 UID: 0 PID: 1 Comm: systemd Tainted: G D 6.16.0-rc2 #1 VOLUNTARY
Tainted: [D]=DIE
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:down_read_trylock+0xaa/0x3b0
Code: d0 7c 08 84 d2 0f 85 a0 02 00 00 8b 0d df 31 dd 04 85 c9 75 29 48 b8 00 00 00 00 00 fc ff df 48 8d 6b 68 48 89 ea 48 c1 ea 03 <80> 3c 02 00 0f 85 88 02 00 00 48 3b 5b 68 0f 85 53 01 00 00 65 ff
RSP: 0000:ffff8881002ce9b8 EFLAGS: 00010016
RAX: dffffc0000000000 RBX: 0000000000000070 RCX: 0000000000000000
RDX: 000000000000001b RSI: 000000000000000a RDI: 0000000000000070
RBP: 00000000000000d8 R08: 0000000000000001 R09: ffffed107dde49d1
R10: ffff8883eef24e8b R11: ffff8881002cec20 R12: 1ffff11020059d37
R13: 00000000003fff7b R14: ffff8881002cec20 R15: dffffc0000000000
FS: 00007f963f21d940(0000) GS:ffff888458ca6000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f963f5edf71 CR3: 000000010672c000 CR4: 0000000000350ef0
Call Trace:
<TASK>
codetag_trylock_module_list+0xd/0x20
alloc_tag_top_users+0x369/0x4b0
__show_mem+0x1cd/0x6e0
warn_alloc+0x2b1/0x390
__alloc_frozen_pages_noprof+0x12b9/0x21a0
alloc_pages_mpol+0x135/0x3e0
alloc_slab_page+0x82/0xe0
new_slab+0x212/0x240
___slab_alloc+0x82a/0xe00
</TASK>

As David Wang points out, this issue became easier to trigger after commit
780138b12381 ("alloc_tag: check mem_profiling_support in alloc_tag_init").

Before the commit, the issue occurred only when it failed to allocate and
initialize alloc_tag_cttype or if a memory allocation fails before
alloc_tag_init() is called. After the commit, it can be easily triggered
when memory profiling is compiled but disabled at boot.

To properly determine whether alloc_tag_init() has been called and its
data structures initialized, verify that alloc_tag_cttype is a valid
pointer before acquiring the semaphore. If the variable is NULL or an
error value, it has not been properly initialized. In such a case, just
skip and do not attempt to acquire the semaphore.

[harry.yoo@oracle.com: v3]