random: remove CONFIG_ARCH_RANDOM

When RDRAND was introduced, there was much discussion on whether it
should be trusted and how the kernel should handle that. Initially, two
mechanisms cropped up, CONFIG_ARCH_RANDOM, a compile time switch, and
"nordrand", a boot-time switch.

Later the thinking evolved. With a properly designed RNG, using RDRAND
values alone won't harm anything, even if the outputs are malicious.
Rather, the issue is whether those values are being *trusted* to be good
or not. And so a new set of options were introduced as the real
ones that people use -- CONFIG_RANDOM_TRUST_CPU and "random.trust_cpu".
With these options, RDRAND is used, but it's not always credited. So in
the worst case, it does nothing, and in the best case, maybe it helps.

Along the way, CONFIG_ARCH_RANDOM's meaning got sort of pulled into the
center and became something certain platforms force-select.

The old options don't really help with much, and it's a bit odd to have
special handling for these instructions when the kernel can deal fine
with the existence or untrusted existence or broken existence or
non-existence of that CPU capability.

Simplify the situation by removing CONFIG_ARCH_RANDOM and using the
ordinary asm-generic fallback pattern instead, keeping the two options
that are actually used. For now it leaves "nordrand" for now, as the
removal of that will take a different route.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Borislav Petkov <bp@suse.de>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

5 files changed, 1 insertions, 22 deletions

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 8cd9e56c629b..9b6e4e7cb17b 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -507,21 +507,6 @@ config KEXEC_SIG
 	  verification for the corresponding kernel image type being
 	  loaded in order for this to work.
 
-config ARCH_RANDOM
-	def_bool y
-	prompt "s390 architectural random number generation API"
-	help
-	  Enable the s390 architectural random number generation API
-	  to provide random data for all consumers within the Linux
-	  kernel.
-
-	  When enabled the arch_random_* functions declared in linux/random.h
-	  are implemented. The implementation is based on the s390 CPACF
-	  instruction subfunction TRNG which provides a real true random
-	  number generator.
-
-	  If unsure, say Y.
-
 config KERNEL_NOBP
 	def_bool n
 	prompt "Enable modified branch prediction for the kernel by default"
diff --git a/arch/s390/configs/zfcpdump_defconfig b/arch/s390/configs/zfcpdump_defconfig
index a87fcc45e307..f4976f611b94 100644
--- a/arch/s390/configs/zfcpdump_defconfig
+++ b/arch/s390/configs/zfcpdump_defconfig
@@ -15,7 +15,6 @@ CONFIG_TUNE_ZEC12=y
 # CONFIG_COMPAT is not set
 CONFIG_NR_CPUS=2
 CONFIG_HZ_100=y
-# CONFIG_ARCH_RANDOM is not set
 # CONFIG_RELOCATABLE is not set
 # CONFIG_CHSC_SCH is not set
 # CONFIG_SCM_BUS is not set
diff --git a/arch/s390/crypto/Makefile b/arch/s390/crypto/Makefile
index c63abfeb6d17..1b1cc478fa94 100644
--- a/arch/s390/crypto/Makefile
+++ b/arch/s390/crypto/Makefile
@@ -15,7 +15,7 @@ obj-$(CONFIG_CRYPTO_CHACHA_S390) += chacha_s390.o
 obj-$(CONFIG_S390_PRNG) += prng.o
 obj-$(CONFIG_CRYPTO_GHASH_S390) += ghash_s390.o
 obj-$(CONFIG_CRYPTO_CRC32_S390) += crc32-vx_s390.o
-obj-$(CONFIG_ARCH_RANDOM) += arch_random.o
+obj-y += arch_random.o
 
 crc32-vx_s390-y := crc32-vx.o crc32le-vx.o crc32be-vx.o
 chacha_s390-y := chacha-glue.o chacha-s390.o
diff --git a/arch/s390/include/asm/archrandom.h b/arch/s390/include/asm/archrandom.h
index 2c6e1c6ecbe7..0a1c2e66c709 100644
--- a/arch/s390/include/asm/archrandom.h
+++ b/arch/s390/include/asm/archrandom.h
@@ -11,8 +11,6 @@
 #ifndef _ASM_S390_ARCHRANDOM_H
 #define _ASM_S390_ARCHRANDOM_H
 
-#ifdef CONFIG_ARCH_RANDOM
-
 #include <linux/static_key.h>
 #include <linux/atomic.h>
 #include <asm/cpacf.h>
@@ -50,5 +48,4 @@ static inline bool __must_check arch_get_random_seed_int(unsigned int *v)
 	return false;
 }
 
-#endif /* CONFIG_ARCH_RANDOM */
 #endif /* _ASM_S390_ARCHRANDOM_H */
diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
index 0a37f5de2863..ebad41afe355 100644
--- a/arch/s390/kernel/setup.c
+++ b/arch/s390/kernel/setup.c
@@ -876,10 +876,8 @@ static void __init setup_randomness(void)
 		add_device_randomness(&vmms->vm, sizeof(vmms->vm[0]) * vmms->count);
 	memblock_free(vmms, PAGE_SIZE);
 
-#ifdef CONFIG_ARCH_RANDOM
 	if (cpacf_query_func(CPACF_PRNO, CPACF_PRNO_TRNG))
 		static_branch_enable(&s390_arch_random_available);
-#endif
 }
 
 /*