linux-kernel - [PATCH v3] KEYS: trusted: allow module init if TPM is inactive or deactivated

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Date:   Mon, 5 Aug 2019 18:44:27 +0200
From:   Roberto Sassu <roberto.sassu@...wei.com>
To:     <jarkko.sakkinen@...ux.intel.com>, <jejb@...ux.ibm.com>,
        <zohar@...ux.ibm.com>, <jgg@...pe.ca>, <tyhicks@...onical.com>
CC:     <linux-integrity@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>,
        <keyrings@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <crazyt2019+lml@...il.com>, <nayna@...ux.vnet.ibm.com>,
        <silviu.vlasceanu@...wei.com>,
        Roberto Sassu <roberto.sassu@...wei.com>
Subject: [PATCH v3] KEYS: trusted: allow module init if TPM is inactive or deactivated

Commit c78719203fc6 ("KEYS: trusted: allow trusted.ko to initialize w/o a
TPM") allows the trusted module to be loaded even if a TPM is not found, to
avoid module dependency problems.

However, trusted module initialization can still fail if the TPM is
inactive or deactivated. tpm_get_random() returns an error.

This patch removes the call to tpm_get_random() and instead extends the PCR
specified by the user with zeros. The security of this alternative is
equivalent to the previous one, as either option prevents with a PCR update
unsealing and misuse of sealed data by a user space process.

Even if a PCR is extended with zeros, instead of random data, it is still
computationally infeasible to find a value as input for a new PCR extend
operation, to obtain again the PCR value that would allow unsealing.

Fixes: 240730437deb ("KEYS: trusted: explicitly use tpm_chip structure...")
Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
Reviewed-by: Tyler Hicks <tyhicks@...onical.com>
Suggested-by: Mimi Zohar <zohar@...ux.ibm.com>
---
 security/keys/trusted.c | 13 -------------
 1 file changed, 13 deletions(-)

diff --git a/security/keys/trusted.c b/security/keys/trusted.c
index 9a94672e7adc..ade699131065 100644
--- a/security/keys/trusted.c
+++ b/security/keys/trusted.c
@@ -1228,24 +1228,11 @@ static int __init trusted_shash_alloc(void)

 static int __init init_digests(void)
 {
-	u8 digest[TPM_MAX_DIGEST_SIZE];
-	int ret;
-	int i;
-
-	ret = tpm_get_random(chip, digest, TPM_MAX_DIGEST_SIZE);
-	if (ret < 0)
-		return ret;
-	if (ret < TPM_MAX_DIGEST_SIZE)
-		return -EFAULT;
-
 	digests = kcalloc(chip->nr_allocated_banks, sizeof(*digests),
 			  GFP_KERNEL);
 	if (!digests)
 		return -ENOMEM;

-	for (i = 0; i < chip->nr_allocated_banks; i++)
-		memcpy(digests[i].digest, digest, TPM_MAX_DIGEST_SIZE);
-
 	return 0;
 }

-- 
2.17.1