crypto: rng - Only allow the DRBG to register as "stdrng" in FIPS mode

In FIPS mode, the DRBG must take precedence over all stdrng algorithms.
The only problem standing in the way of this is that a different stdrng
algorithm could get registered and utilized before the DRBG is registered,
and since crypto_alloc_rng() only allocates an stdrng algorithm when
there's no existing allocation, this means that it's possible for the wrong
stdrng algorithm to remain in use indefinitely.

This issue is also often impossible to observe from userspace; an RNG other
than the DRBG could be used somewhere in the kernel and userspace would be
none the wiser.

To ensure this can never happen, only allow stdrng instances from the DRBG
to be registered when running in FIPS mode. This works since the previous
commit forces the DRBG to be built into the kernel when CONFIG_CRYPTO_FIPS
is enabled, so the DRBG's presence is guaranteed when fips_enabled is true.

Signed-off-by: Sultan Alsawaf <sultan@ciq.com>
Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: kerneltoast

crypto/rng.c

@@ -32,6 +32,7 @@ static ____cacheline_aligned_in_smp DEFINE_MUTEX(crypto_default_rng_lock);
 struct crypto_rng *crypto_default_rng;
 EXPORT_SYMBOL_GPL(crypto_default_rng);
 static unsigned int crypto_default_rng_refcnt;
+static bool drbg_registered __ro_after_init;
 
 int crypto_rng_reset(struct crypto_rng *tfm, const u8 *seed, unsigned int slen)
 {
@@ -201,6 +202,19 @@ int crypto_register_rng(struct rng_alg *alg)
 	if (alg->seedsize > PAGE_SIZE / 8)
 		return -EINVAL;
 
+	/*
+	 * In FIPS mode, the DRBG must take precedence over all other "stdrng"
+	 * algorithms. Therefore, forbid registration of a non-DRBG stdrng in
+	 * FIPS mode. All of the DRBG's driver names are prefixed with "drbg_".
+	 * This also stops new stdrng instances from getting registered after it
+	 * is known that the DRBG is registered, so a new module can't come in
+	 * and pretend to be the DRBG. And when CONFIG_CRYPTO_FIPS is enabled,
+	 * the DRBG is built into the kernel directly; it can't be a module.
+	 */
+	if (fips_enabled && !strcmp(base->cra_name, "stdrng") &&
+	    (drbg_registered || strncmp(base->cra_driver_name, "drbg_", 5)))
+		return -EINVAL;
+
 	base->cra_type = &crypto_rng_type;
 	base->cra_flags &= ~CRYPTO_ALG_TYPE_MASK;
 	base->cra_flags |= CRYPTO_ALG_TYPE_RNG;
@@ -225,6 +239,18 @@ int crypto_register_rngs(struct rng_alg *algs, int count)
 			goto err;
 	}
 
+	/*
+	 * Track when the DRBG is registered in FIPS mode. The DRBG calls
+	 * crypto_register_rngs() to register its stdrng instances, and since
+	 * crypto_register_rng() only allows stdrng instances from the DRBG in
+	 * FIPS mode, a successful stdrng registration means it was the DRBG.
+	 * Just check the first alg in the array to see if it's called "stdrng",
+	 * since all of the DRBG's algorithms are named "stdrng". Once
+	 * drbg_registered is set to true, this if-statement is always false.
+	 */
+	if (fips_enabled && !strcmp(algs->base.cra_name, "stdrng"))
+		drbg_registered = true;
+
 	return 0;
 
 err: