From: Yann Droneaud Subject: Re: [PATCH] random: add a config option to trust the CPU's hwrng Date: Wed, 18 Jul 2018 09:22:13 +0200 Message-ID: <37046662f2b38f98854abfa1b5868a27c3fa0888.camel@opteya.com> References: <20180718014344.1309-1-tytso@mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Cc: labbott@redhat.com To: Theodore Ts'o , linux-crypto@vger.kernel.org, Linux Kernel Developers List Return-path: In-Reply-To: <20180718014344.1309-1-tytso@mit.edu> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-crypto.vger.kernel.org Hi, Le mardi 17 juillet 2018 à 21:43 -0400, Theodore Ts'o a écrit : > This gives the user building their own kernel (or a Linux > distribution) the option of deciding whether or not to trust the CPU's > hardware random number generator (e.g., RDRAND for x86 CPU's) as being > correctly implemented and not having a back door introduced (perhaps > courtesy of a Nation State's law enforcement or intelligence > agencies). > A build time option (RANDOM_TRUST_CPU) is good, but I would also see a kernel command line boot option for the user to disable the behavior whenever its distro kernel was built with RANDOM_TRUST_CPU. It would allow the end-user to make its own decision regarding CPU based hwrng. In particular, during distro installation, end-user doesn't have a chance to rebuild the kernel beforehand, but he can tweak kernel command line during distro installation boot. > This will prevent getrandom(2) from blocking, if there is a > willingness to trust the CPU manufacturer. It would block during initialization / early boot. > > Signed-off-by: Theodore Ts'o > --- > > I'm not sure Linux distro's will thank us for this. The problem is > trusting the CPU manfuacturer can be an emotional / political issue. > > For example, assume that China has decided that as a result of the > "death sentence" that the US government threatened to impose on ZTE > after they were caught introducing privacy violating malware on US > comsumers, that they needed to be self-sufficient in their technology > sector, and so they decided the needed to produce their own CPU. > > Even if I were convinced that Intel hadn't backdoored RDRAND (or an > NSA agent backdoored RDRAND for them) such that the NSA had a NOBUS > (nobody but us) capability to crack RDRAND generated numbers, if we > made a change to unconditionally trust RDRAND, then I didn't want the > upstream kernel developers to have to answer the question, "why are > you willing to trust Intel, but you aren't willing to trust a company > owned and controlled by a PLA general?" (Or a company owned and > controlled by one of Putin's Oligarchs, if that makes you feel > better.) > > With this patch, we don't put ourselves in this position --- but we > do put the Linux distro's in this position intead. The upside is it > gives the choice to each person building their own Linux kernel to > decide whether trusting RDRAND is worth it to avoid hangs due to > userspace trying to get cryptographic-grade entropy early in the boot > process. (Note: I trust RDRAND more than I do Jitter Entropy.) > > drivers/char/Kconfig | 14 ++++++++++++++ > drivers/char/random.c | 11 ++++++++++- > 2 files changed, 24 insertions(+), 1 deletion(-) > > diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig > index 212f447938ae..fe2930c4ecf0 100644 > --- a/drivers/char/Kconfig > +++ b/drivers/char/Kconfig > @@ -554,3 +554,17 @@ config ADI > > endmenu > > +config RANDOM_TRUST_CPU > + bool "Trust the CPU manufacturer to initialize Linux's CRNG" > + depends on (X86 || X86_64 || X86_32 || S390 || PPC) > + default n > + help > + Assume that CPU manufacurer (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 indepedently audited, this amounts to trusting > + that CPU manufacturer (perhaps with the insistance or requirement > + 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. > + The text message should explain this is only relevant during initialization / early boot. The config option name should state this. > diff --git a/drivers/char/random.c b/drivers/char/random.c > index 34ddfd57419b..f4013b8a711b 100644 > --- a/drivers/char/random.c > +++ b/drivers/char/random.c > @@ -782,6 +782,7 @@ static void invalidate_batched_entropy(void); > static void crng_initialize(struct crng_state *crng) > { > int i; > + int arch_init = 1; > unsigned long rv; > > memcpy(&crng->state[0], "expand 32-byte k", 16); > @@ -792,10 +793,18 @@ static void crng_initialize(struct crng_state *crng) > _get_random_bytes(&crng->state[4], sizeof(__u32) * 12); > for (i = 4; i < 16; i++) { > if (!arch_get_random_seed_long(&rv) && > - !arch_get_random_long(&rv)) > + !arch_get_random_long(&rv)) { > rv = random_get_entropy(); > + arch_init = 0; > + } > crng->state[i] ^= rv; > } > +#ifdef CONFIG_RANDOM_TRUST_CPU > + if (arch_init) { > + crng_init = 2; > + pr_notice("random: crng done (trusting CPU's manufacturer)\n"); > + } > +#endif > crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1; > } > Regards. -- Yann Droneaud OPTEYA