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Subject[PATCH 4.19 198/234] random: credit cpu and bootloader seeds by default
Date
From: Jason A. Donenfeld <Jason@zx2c4.com>

[ Upstream commit 846bb97e131d7938847963cca00657c995b1fce1 ]

This commit changes the default Kconfig values of RANDOM_TRUST_CPU and
RANDOM_TRUST_BOOTLOADER to be Y by default. It does not change any
existing configs or change any kernel behavior. The reason for this is
several fold.

As background, I recently had an email thread with the kernel
maintainers of Fedora/RHEL, Debian, Ubuntu, Gentoo, Arch, NixOS, Alpine,
SUSE, and Void as recipients. I noted that some distros trust RDRAND,
some trust EFI, and some trust both, and I asked why or why not. There
wasn't really much of a "debate" but rather an interesting discussion of
what the historical reasons have been for this, and it came up that some
distros just missed the introduction of the bootloader Kconfig knob,
while another didn't want to enable it until there was a boot time
switch to turn it off for more concerned users (which has since been
added). The result of the rather uneventful discussion is that every
major Linux distro enables these two options by default.

While I didn't have really too strong of an opinion going into this
thread -- and I mostly wanted to learn what the distros' thinking was
one way or another -- ultimately I think their choice was a decent
enough one for a default option (which can be disabled at boot time).
I'll try to summarize the pros and cons:

Pros:

- The RNG machinery gets initialized super quickly, and there's no
messing around with subsequent blocking behavior.

- The bootloader mechanism is used by kexec in order for the prior
kernel to initialize the RNG of the next kernel, which increases
the entropy available to early boot daemons of the next kernel.

- Previous objections related to backdoors centered around
Dual_EC_DRBG-like kleptographic systems, in which observing some
amount of the output stream enables an adversary holding the right key
to determine the entire output stream.

This used to be a partially justified concern, because RDRAND output
was mixed into the output stream in varying ways, some of which may
have lacked pre-image resistance (e.g. XOR or an LFSR).

But this is no longer the case. Now, all usage of RDRAND and
bootloader seeds go through a cryptographic hash function. This means
that the CPU would have to compute a hash pre-image, which is not
considered to be feasible (otherwise the hash function would be
terribly broken).

- More generally, if the CPU is backdoored, the RNG is probably not the
realistic vector of choice for an attacker.

- These CPU or bootloader seeds are far from being the only source of
entropy. Rather, there is generally a pretty huge amount of entropy,
not all of which is credited, especially on CPUs that support
instructions like RDRAND. In other words, assuming RDRAND outputs all
zeros, an attacker would *still* have to accurately model every single
other entropy source also in use.

- The RNG now reseeds itself quite rapidly during boot, starting at 2
seconds, then 4, then 8, then 16, and so forth, so that other sources
of entropy get used without much delay.

- Paranoid users can set random.trust_{cpu,bootloader}=no in the kernel
command line, and paranoid system builders can set the Kconfig options
to N, so there's no reduction or restriction of optionality.

- It's a practical default.

- All the distros have it set this way. Microsoft and Apple trust it
too. Bandwagon.

Cons:

- RDRAND *could* still be backdoored with something like a fixed key or
limited space serial number seed or another indexable scheme like
that. (However, it's hard to imagine threat models where the CPU is
backdoored like this, yet people are still okay making *any*
computations with it or connecting it to networks, etc.)

- RDRAND *could* be defective, rather than backdoored, and produce
garbage that is in one way or another insufficient for crypto.

- Suggesting a *reduction* in paranoia, as this commit effectively does,
may cause some to question my personal integrity as a "security
person".

- Bootloader seeds and RDRAND are generally very difficult if not all
together impossible to audit.

Keep in mind that this doesn't actually change any behavior. This
is just a change in the default Kconfig value. The distros already are
shipping kernels that set things this way.

Ard made an additional argument in [1]:

We're at the mercy of firmware and micro-architecture anyway, given
that we are also relying on it to ensure that every instruction in
the kernel's executable image has been faithfully copied to memory,
and that the CPU implements those instructions as documented. So I
don't think firmware or ISA bugs related to RNGs deserve special
treatment - if they are broken, we should quirk around them like we
usually do. So enabling these by default is a step in the right
direction IMHO.

In [2], Phil pointed out that having this disabled masked a bug that CI
otherwise would have caught:

A clean 5.15.45 boots cleanly, whereas a downstream kernel shows the
static key warning (but it does go on to boot). The significant
difference is that our defconfigs set CONFIG_RANDOM_TRUST_BOOTLOADER=y
defining that on top of multi_v7_defconfig demonstrates the issue on
a clean 5.15.45. Conversely, not setting that option in a
downstream kernel build avoids the warning

[1] https://lore.kernel.org/lkml/CAMj1kXGi+ieviFjXv9zQBSaGyyzeGW_VpMpTLJK8PJb2QHEQ-w@mail.gmail.com/
[2] https://lore.kernel.org/lkml/c47c42e3-1d56-5859-a6ad-976a1a3381c6@raspberrypi.com/

Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/char/Kconfig | 54 +++++++++++++++++++++++++++-----------------
1 file changed, 33 insertions(+), 21 deletions(-)

diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 094548183f8b..f55c9bbd58fb 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -552,29 +552,41 @@ config ADI
and SSM (Silicon Secured Memory). Intended consumers of this
driver include crash and makedumpfile.

-endmenu
-
config RANDOM_TRUST_CPU
- bool "Trust the CPU manufacturer to initialize Linux's CRNG"
- depends on X86 || S390 || PPC
- default n
+ bool "Initialize RNG using CPU RNG instructions"
+ default y
+ depends on ARCH_RANDOM
help
- Assume that CPU manufacturer (e.g., Intel or AMD for RDSEED or
- RDRAND, IBM for the S390 and Power PC architectures) is trustworthy
- for the purposes of initializing Linux's CRNG. Since this is not
- something that can be independently audited, this amounts to trusting
- that CPU manufacturer (perhaps with the insistence or mandate
- of a Nation State's intelligence or law enforcement agencies)
- has not installed a hidden back door to compromise the CPU's
- random number generation facilities. This can also be configured
- at boot with "random.trust_cpu=on/off".
+ Initialize the RNG using random numbers supplied by the CPU's
+ RNG instructions (e.g. RDRAND), if supported and available. These
+ random numbers are never used directly, but are rather hashed into
+ the main input pool, and this happens regardless of whether or not
+ this option is enabled. Instead, this option controls whether the
+ they are credited and hence can initialize the RNG. Additionally,
+ other sources of randomness are always used, regardless of this
+ setting. Enabling this implies trusting that the CPU can supply high
+ quality and non-backdoored random numbers.
+
+ Say Y here unless you have reason to mistrust your CPU or believe
+ its RNG facilities may be faulty. This may also be configured at
+ boot time with "random.trust_cpu=on/off".

config RANDOM_TRUST_BOOTLOADER
- bool "Trust the bootloader to initialize Linux's CRNG"
+ bool "Initialize RNG using bootloader-supplied seed"
+ default y
help
- Some bootloaders can provide entropy to increase the kernel's initial
- device randomness. Say Y here to assume the entropy provided by the
- booloader is trustworthy so it will be added to the kernel's entropy
- pool. Otherwise, say N here so it will be regarded as device input that
- only mixes the entropy pool. This can also be configured at boot with
- "random.trust_bootloader=on/off".
+ Initialize the RNG using a seed supplied by the bootloader or boot
+ environment (e.g. EFI or a bootloader-generated device tree). This
+ seed is not used directly, but is rather hashed into the main input
+ pool, and this happens regardless of whether or not this option is
+ enabled. Instead, this option controls whether the seed is credited
+ and hence can initialize the RNG. Additionally, other sources of
+ randomness are always used, regardless of this setting. Enabling
+ this implies trusting that the bootloader can supply high quality and
+ non-backdoored seeds.
+
+ Say Y here unless you have reason to mistrust your bootloader or
+ believe its RNG facilities may be faulty. This may also be configured
+ at boot time with "random.trust_bootloader=on/off".
+
+endmenu
--
2.35.1


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