KEYS: trusted: Fix TPM teardown ordering

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

KEYS: trusted: Fix TPM teardown ordering

trusted_tpm_exit() drops the TPM chip reference and frees the digest
array before unregistering the trusted key type. key_type_lookup()
holds key_types_sem for reading until the key operation finishes, while
unregister_key_type() takes it for writing. It therefore provides the
synchronization point that must precede backend teardown.

The current order permits this interleaving:

CPU 0 CPU 1
trusted_tpm_exit() key_type_lookup("trusted")
put_device(&chip->dev) trusted_tpm_seal()
kfree(digests) pcrlock()
unregister_key_type() tpm_pcr_extend(..., digests)

CPU 1 can consequently dereference the freed digest array. The chip can
also be released before callbacks stop using it.

KASAN reported:

BUG: KASAN: slab-use-after-free in tpm_pcr_extend+0x1f0/0x200
Read of size 2 at addr ffff88810872d000 by task poc/89
Call Trace:
tpm_pcr_extend+0x1f0/0x200
pcrlock+0x42/0x70 [trusted]
trusted_tpm_seal+0x1b6/0x570 [trusted]
trusted_instantiate+0x293/0x340 [trusted]
__key_instantiate_and_link+0xb2/0x2b0
__key_create_or_update+0x61e/0xb50
__do_sys_add_key+0x1b8/0x310
Allocated by task 88:
__kmalloc_noprof+0x1a7/0x490
do_one_initcall+0xa1/0x390
do_init_module+0x2df/0x840
Freed by task 90:
kfree+0x131/0x3c0
trusted_tpm_exit+0x59/0xa0 [trusted]
__do_sys_delete_module+0x346/0x510

Move unregister_key_type() before releasing either resource. This stops
new lookups and waits for in-flight key operations to finish before the
backend state is destroyed.