[PATCH] KEYS: trusted: Fix TPM teardown ordering

[PATCH] KEYS: trusted: Fix TPM teardown ordering

[Chengfeng Ye]

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.

Fixes: 0b6cf6b97b7e ("tpm: pass an array of tpm_extend_digest structures to tpm_pcr_extend()")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 security/keys/trusted-keys/trusted_tpm1.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/keys/trusted-keys/trusted_tpm1.c b/security/keys/trusted-keys/trusted_tpm1.c
index 13513819991e..8f57c6111e7e 100644
--- a/security/keys/trusted-keys/trusted_tpm1.c
+++ b/security/keys/trusted-keys/trusted_tpm1.c
@@ -987,9 +987,9 @@ static int __init trusted_tpm_init(void)
 static void trusted_tpm_exit(void)
 {
 	if (chip) {
+		unregister_key_type(&key_type_trusted);
 		put_device(&chip->dev);
 		kfree(digests);
-		unregister_key_type(&key_type_trusted);
 	}
 }
 
-- 
2.43.0