Re: [PATCH 5/8] security: keys: trusted: Make sealed key properly interoperable

[David Woodhouse <dwmw2@infradead.org>]

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On Sat, 2019-12-07 at 21:11 -0800, James Bottomley wrote:
> The current implementation appends a migratable flag to the end of a
> key, meaning the format isn't exactly interoperable because the using
> party needs to know to strip this extra byte.  However, all other
> consumers of TPM sealed blobs expect the unseal to return exactly the
> key.  Since TPM2 keys have a key property flag that corresponds to
> migratable, use that flag instead and make the actual key the only
> sealed quantity.  This is secure because the key properties are bound
> to a hash in the private part, so if they're altered the key won't
> load.
> 
> Backwards compatibility is implemented by detecting whether we're
> loading a new format key or not and correctly setting migratable from
> the last byte of old format keys.
> 
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
> ---
>  include/keys/trusted-type.h               |  1 +
>  include/linux/tpm.h                       |  2 ++
>  security/keys/trusted-keys/trusted_tpm2.c | 57 ++++++++++++++++++++++---------
>  3 files changed, 44 insertions(+), 16 deletions(-)
> 
> diff --git a/include/keys/trusted-type.h b/include/keys/trusted-type.h
> index a94c03a61d8f..4728e13aada8 100644
> --- a/include/keys/trusted-type.h
> +++ b/include/keys/trusted-type.h
> @@ -22,6 +22,7 @@ struct trusted_key_payload {
>  	unsigned int key_len;
>  	unsigned int blob_len;
>  	unsigned char migratable;
> +	unsigned char old_format;
>  	unsigned char key[MAX_KEY_SIZE + 1];
>  	unsigned char blob[MAX_BLOB_SIZE];
>  };
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index 03e9b184411b..cd46ab27baa5 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -297,6 +297,8 @@ struct tpm_buf {
>  };
>  
>  enum tpm2_object_attributes {
> +	TPM2_OA_FIXED_TPM		= BIT(1),
> +	TPM2_OA_FIXED_PARENT		= BIT(4),
>  	TPM2_OA_USER_WITH_AUTH		= BIT(6),
>  };
>  
> diff --git a/security/keys/trusted-keys/trusted_tpm2.c b/security/keys/trusted-keys/trusted_tpm2.c
> index 4efc7b64d1cd..a34ab6f90f76 100644
> --- a/security/keys/trusted-keys/trusted_tpm2.c
> +++ b/security/keys/trusted-keys/trusted_tpm2.c
> @@ -207,6 +207,7 @@ int tpm2_seal_trusted(struct tpm_chip *chip,
>  	unsigned int blob_len;
>  	struct tpm_buf buf;
>  	u32 hash;
> +	u32 flags;
>  	int i;
>  	int rc;
>  
> @@ -235,29 +236,30 @@ int tpm2_seal_trusted(struct tpm_chip *chip,
>  			     TPM_DIGEST_SIZE);
>  
>  	/* sensitive */
> -	tpm_buf_append_u16(&buf, 4 + TPM_DIGEST_SIZE + payload->key_len + 1);
> +	tpm_buf_append_u16(&buf, 4 + TPM_DIGEST_SIZE + payload->key_len);
>  
>  	tpm_buf_append_u16(&buf, TPM_DIGEST_SIZE);
>  	tpm_buf_append(&buf, options->blobauth, TPM_DIGEST_SIZE);
> -	tpm_buf_append_u16(&buf, payload->key_len + 1);
> +	tpm_buf_append_u16(&buf, payload->key_len);
>  	tpm_buf_append(&buf, payload->key, payload->key_len);
> -	tpm_buf_append_u8(&buf, payload->migratable);
>  
>  	/* public */
>  	tpm_buf_append_u16(&buf, 14 + options->policydigest_len);
>  	tpm_buf_append_u16(&buf, TPM_ALG_KEYEDHASH);
>  	tpm_buf_append_u16(&buf, hash);
>  
> +	/* key properties */
> +	flags = 0;
> +	flags |= options->policydigest_len ? 0 : TPM2_OA_USER_WITH_AUTH;
> +	flags |= payload->migratable ? (TPM2_OA_FIXED_TPM |
> +					TPM2_OA_FIXED_PARENT) : 0;
> +	tpm_buf_append_u32(&buf, flags);
> +
>  	/* policy */
> -	if (options->policydigest_len) {
> -		tpm_buf_append_u32(&buf, 0);
> -		tpm_buf_append_u16(&buf, options->policydigest_len);
> +	tpm_buf_append_u16(&buf, options->policydigest_len);
> +	if (options->policydigest_len)
>  		tpm_buf_append(&buf, options->policydigest,
>  			       options->policydigest_len);
> -	} else {
> -		tpm_buf_append_u32(&buf, TPM2_OA_USER_WITH_AUTH);
> -		tpm_buf_append_u16(&buf, 0);
> -	}
>  
>  	/* public parameters */
>  	tpm_buf_append_u16(&buf, TPM_ALG_NULL);
> @@ -330,13 +332,16 @@ static int tpm2_load_cmd(struct tpm_chip *chip,
>  	unsigned int private_len;
>  	unsigned int public_len;
>  	unsigned int blob_len;
> -	u8 *blob;
> +	u8 *blob, *pub;
>  	int rc;
> +	u32 attrs;
>  
>  	rc = tpm2_key_decode(payload, options, &blob);
> -	if (rc)
> +	if (rc) {
>  		/* old form */
>  		blob = payload->blob;
> +		payload->old_format = 1;
> +	}
>  
>  	/* new format carries keyhandle but old format doesn't */
>  	if (!options->keyhandle)
> @@ -347,6 +352,16 @@ static int tpm2_load_cmd(struct tpm_chip *chip,
>  		return -E2BIG;
>  
>  	public_len = be16_to_cpup((__be16 *) &blob[2 + private_len]);
> +
> +	pub = blob + 2 + private_len + 2;
> +	/* key attributes are always at offset 4 */
> +	attrs = get_unaligned_be32(pub + 4);


At this point I don't believe you've checked yet that payload->blob_len 
is sufficient to know that these bytes exist.

I think you're reading 'private_len' from non-existent bytes too, if
payload->blob_len is zero or one? Which I think was there before you
started, but you touched it last...


> +	if ((attrs & (TPM2_OA_FIXED_TPM | TPM2_OA_FIXED_PARENT)) ==
> +	    (TPM2_OA_FIXED_TPM | TPM2_OA_FIXED_PARENT))
> +		payload->migratable = 0;
> +	else
> +		payload->migratable = 1;
> +
>  	blob_len = private_len + public_len + 4;
>  	if (blob_len > payload->blob_len)
>  		return -E2BIG;
> @@ -427,7 +442,7 @@ static int tpm2_unseal_cmd(struct tpm_chip *chip,
>  	if (!rc) {
>  		data_len = be16_to_cpup(
>  			(__be16 *) &buf.data[TPM_HEADER_SIZE + 4]);
> -		if (data_len < MIN_KEY_SIZE ||  data_len > MAX_KEY_SIZE + 1) {
> +		if (data_len < MIN_KEY_SIZE ||  data_len > MAX_KEY_SIZE) {
>  			rc = -EFAULT;
>  			goto out;
>  		}
> @@ -438,9 +453,19 @@ static int tpm2_unseal_cmd(struct tpm_chip *chip,
>  		}
>  		data = &buf.data[TPM_HEADER_SIZE + 6];
>  
> -		memcpy(payload->key, data, data_len - 1);
> -		payload->key_len = data_len - 1;
> -		payload->migratable = data[data_len - 1];
> +		if (payload->old_format) {
> +			/* migratable flag is at the end of the key */
> +			memcpy(payload->key, data, data_len - 1);
> +			payload->key_len = data_len - 1;
> +			payload->migratable = data[data_len - 1];
> +		} else {
> +			/*
> +			 * migratable flag already collected from key
> +			 * attributes
> +			 */
> +			memcpy(payload->key, data, data_len);
> +			payload->key_len = data_len;
> +		}
>  	}
>  
>  out:


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