crypto: Kconfig - Make CRYPTO_FIPS depend on the DRBG being built-in

When FIPS mode is enabled (via fips=1), there is an absolute need for the
DRBG to be available. This is at odds with the fact that the DRBG can be
built as a module when in FIPS mode, leaving critical RNG functionality at
the whims of userspace.

Userspace could simply rmmod the DRBG module, or not provide it at all and
thus a different stdrng algorithm could be used without anyone noticing.

Additionally, when running a FIPS-enabled userspace, modprobe itself may
perform a getrandom() syscall _before_ loading a given module. As a result,
there's a possible deadlock scenario where the RNG core (crypto/rng.c)
initializes _before_ the DRBG, thereby installing its getrandom() override
without having an stdrng algorithm available. Then, when userspace calls
getrandom() which redirects to the override in crypto/rng.c,
crypto_alloc_rng("stdrng") invokes the UMH (modprobe) to load the DRBG
(which is aliased to stdrng). And *then* that modprobe invocation gets
stuck at getrandom() because there's no stdrng algorithm available!

There are too many risks that come with allowing the DRBG and RNG core to
be modular for FIPS mode. Therefore, make CRYPTO_FIPS require the DRBG to
be built-in, which in turn makes the DRBG require the RNG core to be
built-in. That way, it's guaranteed for these drivers to be built-in when
running in FIPS mode.

Also clean up the CRYPTO_FIPS option name and remove the CRYPTO_ANSI_CPRNG
dependency since it's obsolete for FIPS now.

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

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

Author: kerneltoast

crypto/Kconfig

@@ -23,12 +23,12 @@ if CRYPTO
 comment "Crypto core or helper"
 
 config CRYPTO_FIPS
-	bool "FIPS 200 compliance"
-	depends on (CRYPTO_ANSI_CPRNG || CRYPTO_DRBG) && !CRYPTO_MANAGER_DISABLE_TESTS
+	bool "FIPS compliance"
+	depends on CRYPTO_DRBG=y && !CRYPTO_MANAGER_DISABLE_TESTS
 	depends on (MODULE_SIG || !MODULES)
 	help
 	  This option enables the fips boot option which is
-	  required if you want the system to operate in a FIPS 200
+	  required if you want the system to operate in a FIPS
 	  certification.  You should say no unless you know what
 	  this is.