[PATCH v4 2/5] KEYS: trusted: allow users to use kernel RNG for key material

[Ahmad Fatoum]

The two existing trusted key sources don't make use of the kernel RNG,
but instead let the hardware doing the sealing/unsealing also
generate the random key material. However, Users may want to place
less trust into the quality of the trust source's random number
generator and instead use the kernel entropy pool, which can be
seeded from multiple entropy sources.

Make this possible by adding a new trusted.kernel_rng parameter,
that will force use of the kernel RNG. In its absence, it's up
to the trust source to decide, which random numbers to use,
maintaining the existing behavior.

Suggested-by: Jarkko Sakkinen <jarkko at kernel.org>
Acked-by: Sumit Garg <sumit.garg at linaro.org>
Reviewed-by: David Gstir <david at sigma-star.at>
Tested-By: Tim Harvey <tharvey at gateworks.com>
Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
---
To: James Bottomley <jejb at linux.ibm.com>
To: Jarkko Sakkinen <jarkko at kernel.org>
To: Mimi Zohar <zohar at linux.ibm.com>
To: David Howells <dhowells at redhat.com>
Cc: James Morris <jmorris at namei.org>
Cc: "Serge E. Hallyn" <serge at hallyn.com>
Cc: "Horia Geantă" <horia.geanta at nxp.com>
Cc: Aymen Sghaier <aymen.sghaier at nxp.com>
Cc: Herbert Xu <herbert at gondor.apana.org.au>
Cc: "David S. Miller" <davem at davemloft.net>
Cc: Udit Agarwal <udit.agarwal at nxp.com>
Cc: Jan Luebbe <j.luebbe at pengutronix.de>
Cc: Eric Biggers <ebiggers at kernel.org>
Cc: David Gstir <david at sigma-star.at>
Cc: Richard Weinberger <richard at nod.at>
Cc: Franck LENORMAND <franck.lenormand at nxp.com>
Cc: Sumit Garg <sumit.garg at linaro.org>
Cc: keyrings at vger.kernel.org
Cc: linux-crypto at vger.kernel.org
Cc: linux-integrity at vger.kernel.org
Cc: linux-kernel at vger.kernel.org
Cc: linux-security-module at vger.kernel.org
---
 Documentation/admin-guide/kernel-parameters.txt   |  7 ++++++-
 Documentation/security/keys/trusted-encrypted.rst | 20 +++++++++-------
 security/keys/trusted-keys/trusted_core.c         | 17 +++++++++++++-
 3 files changed, 35 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 43dc35fe5bc0..d5969452f063 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5772,6 +5772,13 @@
 			first trust source as a backend which is initialized
 			successfully during iteration.
 
+	trusted.kernel_rng =	[KEYS]
+			Format: <bool>
+			When set to true (1), the kernel random number pool
+			is used to generate key material for trusted keys.
+			The default is to leave the RNG's choice to each
+			individual trust source.
+
 	tsc=		Disable clocksource stability checks for TSC.
 			Format: <string>
 			[x86] reliable: mark tsc clocksource as reliable, this
diff --git a/Documentation/security/keys/trusted-encrypted.rst b/Documentation/security/keys/trusted-encrypted.rst
index 80d5a5af62a1..1d4b4b8f12f0 100644
--- a/Documentation/security/keys/trusted-encrypted.rst
+++ b/Documentation/security/keys/trusted-encrypted.rst
@@ -87,22 +87,26 @@ Key Generation
 Trusted Keys
 ------------
 
-New keys are created from random numbers generated in the trust source. They
-are encrypted/decrypted using a child key in the storage key hierarchy.
-Encryption and decryption of the child key must be protected by a strong
-access control policy within the trust source.
+New keys are created from random numbers. They are encrypted/decrypted using
+a child key in the storage key hierarchy. Encryption and decryption of the
+child key must be protected by a strong access control policy within the
+trust source. The random number generator in use differs according to the
+selected trust source:
 
-  *  TPM (hardware device) based RNG
+  *  TPM: hardware device based RNG
 
-     Strength of random numbers may vary from one device manufacturer to
-     another.
+     Keys are generated within the TPM. Strength of random numbers may vary
+     from one device manufacturer to another.
 
-  *  TEE (OP-TEE based on Arm TrustZone) based RNG
+  *  TEE: OP-TEE based on Arm TrustZone based RNG
 
      RNG is customizable as per platform needs. It can either be direct output
      from platform specific hardware RNG or a software based Fortuna CSPRNG
      which can be seeded via multiple entropy sources.
 
+Optionally, users may specify ``trusted.kernel_rng=1`` on the kernel
+command-line to override the used RNG with the kernel's random number pool.
+
 Encrypted Keys
 --------------
 
diff --git a/security/keys/trusted-keys/trusted_core.c b/security/keys/trusted-keys/trusted_core.c
index 8cab69e5d0da..569af9af8df0 100644
--- a/security/keys/trusted-keys/trusted_core.c
+++ b/security/keys/trusted-keys/trusted_core.c
@@ -16,12 +16,17 @@
 #include <linux/key-type.h>
 #include <linux/module.h>
 #include <linux/parser.h>
+#include <linux/random.h>
 #include <linux/rcupdate.h>
 #include <linux/slab.h>
 #include <linux/static_call.h>
 #include <linux/string.h>
 #include <linux/uaccess.h>
 
+static bool trusted_kernel_rng;
+module_param_named(kernel_rng, trusted_kernel_rng, bool, 0);
+MODULE_PARM_DESC(kernel_rng, "Generate key material from kernel RNG");
+
 static char *trusted_key_source;
 module_param_named(source, trusted_key_source, charp, 0);
 MODULE_PARM_DESC(source, "Select trusted keys source (tpm or tee)");
@@ -312,8 +317,14 @@ struct key_type key_type_trusted = {
 };
 EXPORT_SYMBOL_GPL(key_type_trusted);
 
+static int kernel_get_random(unsigned char *key, size_t key_len)
+{
+	return get_random_bytes_wait(key, key_len) ?: key_len;
+}
+
 static int __init init_trusted(void)
 {
+	int (*get_random)(unsigned char *key, size_t key_len);
 	int i, ret = 0;
 
 	for (i = 0; i < ARRAY_SIZE(trusted_key_sources); i++) {
@@ -322,6 +333,10 @@ static int __init init_trusted(void)
 			    strlen(trusted_key_sources[i].name)))
 			continue;
 
+		get_random = trusted_key_sources[i].ops->get_random;
+		if (trusted_kernel_rng)
+			get_random = kernel_get_random;
+
 		static_call_update(trusted_key_init,
 				   trusted_key_sources[i].ops->init);
 		static_call_update(trusted_key_seal,
@@ -329,7 +344,7 @@ static int __init init_trusted(void)
 		static_call_update(trusted_key_unseal,
 				   trusted_key_sources[i].ops->unseal);
 		static_call_update(trusted_key_get_random,
-				   trusted_key_sources[i].ops->get_random);
+				   get_random);
 		static_call_update(trusted_key_exit,
 				   trusted_key_sources[i].ops->exit);
 		migratable = trusted_key_sources[i].ops->migratable;
-- 
git-series 0.9.1