Re: [PATCH] KEYS: trusted: Fix TPM teardown ordering

On Fri, Jul 31, 2026 at 10:09:24PM +0800, Chengfeng Ye wrote:
> 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.

Thank for the great report.

> 
> Fixes: 0b6cf6b97b7e ("tpm: pass an array of tpm_extend_digest structures to tpm_pcr_extend()")
> Cc: [email protected]
> Signed-off-by: Chengfeng Ye <[email protected]>
> ---
>  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
> 

Reviewed-by: Jarkko Sakkinen <[email protected]>

BR, Jarkko