Re: [PATCH] random: add rng-seed= command line option

[Masami Hiramatsu <mhiramat@kernel.org>]

Hi Mark,

On Thu, 13 Feb 2020 10:44:59 -0800
Mark Salyzyn <salyzyn@android.com> wrote:

> On 2/13/20 7:03 AM, Masami Hiramatsu wrote:
> > On Thu, 13 Feb 2020 20:24:54 +0900
> > Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> >>>> My preference would be to pass in the random seed *not* on the
> >>>> command-line at all, but as a separate parameter which is passed to
> >>>> the bootloader, just as we pass in the device-tree, the initrd and the
> >>>> command-line as separate things.  The problem is that how we pass in
> >>>> extra boot parameters is architecture specific, and how we might do it
> >>>> for x86 is different than for arm64.  So yeah, it's a bit more
> >>>> inconvenient to do things that way; but I think it's also much
> >>>> cleaner.
> >>> Hmm, if the boot loader could add on to the bootconfig that Masami just
> >>> added, then it could add some "random" seed for each boot! The
> >>> bootconfig is just an appended file at the end of the initrd.
> >> Yeah, it is easy to add bootconfig support to a bootloader. It can add
> >> a entropy number as "rng.seed=XXX" text after initrd image with size
> >> and checksum. That is architecutre independent way to pass such hidden
> >> parameter.
> >> (hidden key must be filtered out when printing out the /proc/bootconfig,
> >> but that is very easy too, just need a strncmp)
> >>
> > And here is the patch to support "random.rng_seed = XXX" option by
> > bootconfig. Now you can focus on what you want to do. No need to
> > modify command line strings.
> 
> LGTM, our virtualized emulator (cuttlefish) folks believe they can do it 
> this way. Albeit keep in mind that there are _thousands_ of embedded 
> bootloaders out there that are comfortable updating DT and kernel 
> command line, but few that add boot configs, so this may add complexity. 

I see, since the bootconfig is a new feature, it will take a time to
be supported widely. Even though, maybe they can use DT for that
purpose.

> For our use case that caused us to need this, the cuttlefish Android 
> emulated device, not a problem though.
> 
> However, the entropy _data_ has not been added (see below)

Oh, I missed that.

> 
> Could you please formally re-submit your patch mhiramet@ with a change 
> to push the _data_ as well to the entropy?

Yes, I'll do.

> 
> -- Mark
> 
> >
> > BTW, if you think you need to pass UTF-8 code as a data, I'm happy to
> > update the bootconfig to support it. Just for the safeness, I have
> > limited it by isprint() || isspace().
> >
> > Thank you,
> >
> > diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
> > index 26956c006987..43fbbd307204 100644
> > --- a/drivers/char/Kconfig
> > +++ b/drivers/char/Kconfig
> > @@ -554,6 +554,7 @@ config RANDOM_TRUST_CPU
> >   
> >   config RANDOM_TRUST_BOOTLOADER
> >   	bool "Trust the bootloader to initialize Linux's CRNG"
> > +	select BOOT_CONFIG
> >   	help
> >   	Some bootloaders can provide entropy to increase the kernel's initial
> >   	device randomness. Say Y here to assume the entropy provided by the
> > diff --git a/drivers/char/random.c b/drivers/char/random.c
> > index c7f9584de2c8..0ae33bbbd338 100644
> > --- a/drivers/char/random.c
> > +++ b/drivers/char/random.c
> > @@ -2311,3 +2311,11 @@ void add_bootloader_randomness(const void *buf, unsigned int size)
> >   		add_device_randomness(buf, size);
> >   }
> >   EXPORT_SYMBOL_GPL(add_bootloader_randomness);
> > +
> > +#if defined(CONFIG_RANDOM_TRUST_BOOTLOADER)
> > +/* caller called add_device_randomness, but it is from a trusted source */
> > +void __init credit_trusted_entropy_bits(unsigned int nbits)
> > +{
> > +	credit_entropy_bits(&input_pool, nbits);
> > +}
> > +#endif
> > diff --git a/fs/proc/bootconfig.c b/fs/proc/bootconfig.c
> > index 9955d75c0585..aace466c56ed 100644
> > --- a/fs/proc/bootconfig.c
> > +++ b/fs/proc/bootconfig.c
> > @@ -36,6 +36,9 @@ static int __init copy_xbc_key_value_list(char *dst, size_t size)
> >   		ret = xbc_node_compose_key(leaf, key, XBC_KEYLEN_MAX);
> >   		if (ret < 0)
> >   			break;
> > +		/* For keeping security reason, remove randomness key */
> > +		if (!strcmp(key, RANDOM_SEED_XBC_KEY))
> > +			continue;
> >   		ret = snprintf(dst, rest(dst, end), "%s = ", key);
> >   		if (ret < 0)
> >   			break;
> > diff --git a/include/linux/random.h b/include/linux/random.h
> > index d319f9a1e429..c8f41ab4f342 100644
> > --- a/include/linux/random.h
> > +++ b/include/linux/random.h
> > @@ -20,6 +20,13 @@ struct random_ready_callback {
> >   
> >   extern void add_device_randomness(const void *, unsigned int);
> >   extern void add_bootloader_randomness(const void *, unsigned int);
> > +#if defined(CONFIG_RANDOM_TRUST_BOOTLOADER)
> > +extern void __init credit_trusted_entropy_bits(unsigned int nbits);
> > +#else
> > +static inline void credit_trusted_entropy_bits(unsigned int nbits) {}
> > +#endif
> > +
> > +#define RANDOM_SEED_XBC_KEY "random.rng_seed"
> >   
> >   #if defined(LATENT_ENTROPY_PLUGIN) && !defined(__CHECKER__)
> >   static inline void add_latent_entropy(void)
> > diff --git a/init/main.c b/init/main.c
> > index f95b014a5479..6c3f51bc76d5 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -776,6 +776,32 @@ void __init __weak arch_call_rest_init(void)
> >   	rest_init();
> >   }
> >   
> > +static __always_inline void __init collect_entropy(const char *command_line)
> > +{
> > +	/*
> > +	 * For best initial stack canary entropy, prepare it after:
> > +	 * - setup_arch() for any UEFI RNG entropy and boot cmdline access
> > +	 * - timekeeping_init() for ktime entropy used in rand_initialize()
> > +	 * - rand_initialize() to get any arch-specific entropy like RDRAND
> > +	 * - add_latent_entropy() to get any latent entropy
> > +	 * - adding command line entropy
> > +	 */
> > +	rand_initialize();
> > +	add_latent_entropy();
> > +	add_device_randomness(command_line, strlen(command_line));
> > +	if (IS_BUILTIN(CONFIG_RANDOM_TRUST_BOOTLOADER)) {
> > +		/*
> > +		 * Added bootconfig device randomness above,
> 
> This part is incorrect, the rng_seed collected below was _not_ added to 
> the device_randomness.
> 
> add_device_randomness(rng_seed, strlen(rng_seed)) needs to be pushed 
> below along with the credit.

OK, as same as above command_line, I'll add that.

Thank you,

> 
> > +		 * now add entropy credit for just random.rng_seed=<data>
> > +		 */
> > +		const char *rng_seed = xbc_find_value(RANDOM_SEED_XBC_KEY, NULL);
> > +
> > +		if (rng_seed)
> > +			credit_trusted_entropy_bits(strlen(rng_seed) * 6);
> > +	}
> > +	boot_init_stack_canary();
> > +}
> > +
> >   asmlinkage __visible void __init start_kernel(void)
> >   {
> >   	char *command_line;
> > @@ -887,18 +913,7 @@ asmlinkage __visible void __init start_kernel(void)
> >   	softirq_init();
> >   	timekeeping_init();
> >   
> > -	/*
> > -	 * For best initial stack canary entropy, prepare it after:
> > -	 * - setup_arch() for any UEFI RNG entropy and boot cmdline access
> > -	 * - timekeeping_init() for ktime entropy used in rand_initialize()
> > -	 * - rand_initialize() to get any arch-specific entropy like RDRAND
> > -	 * - add_latent_entropy() to get any latent entropy
> > -	 * - adding command line entropy
> > -	 */
> > -	rand_initialize();
> > -	add_latent_entropy();
> > -	add_device_randomness(command_line, strlen(command_line));
> > -	boot_init_stack_canary();
> > +	collect_entropy(command_line);
> >   
> >   	time_init();
> >   	printk_safe_init();
> >
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>