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

[Mark Salyzyn <salyzyn@android.com>]

On 2/7/20 9:28 AM, Kees Cook wrote:
> On Fri, Feb 07, 2020 at 07:07:59AM -0800, Mark Salyzyn wrote:
>> +#if defined(CONFIG_RANDOM_TRUST_BOOTLOADER)
>> +/* caller called add_device_randomness, but it is from a trusted source */
>> +void __init credit_trusted_entropy(unsigned int size)
>> +{
>> +	credit_entropy_bits(&input_pool, size * 8);
>> +}
>> +#endif
> As Ted already mentioned, I was expecting the string contents to actually
> get added somewhere. Is the idea that it's already been added via the
> add_device_randomness(command_line) call, and you just want to explicitly
> credit those bytes? If so, that deserves a comment, and I think it should
> likely not use 8 bits per character, as that's not how many bits are
> possible for an alphanumeric string character; I would expect 6 bits (~32
> standard letter/number) -- this likely needs fixing in the fdt patch too.

Yup, responded to Ted as such.

Both can have near-raw 8-bit data as long as they stay away from certain 
characters.

For the command line space and nul characters. Since rng-seed is 
stripped out of any views no one needs to get hurt.

For OF some other parse characters need to be skipped. The rng-seed is 
also memset'd out of existence after being read so no one gets hurt.

I see no harm with multiplying by six in both cases as entropy credit 
should be realistic, but generators can be more ambitious ...

> . .  .
>> +}
>> +
>>   static void __init setup_command_line(char *command_line)
>>   {
>>   	size_t len, xlen = 0, ilen = 0;
>> +	static const char argsep_str[] __initconst = " -- ";
>> +	static const char alloc_fail_msg[] __initconst =
>> +		"%s: Failed to allocate %zu bytes\n";
> There's some refactoring in this patch unrelated to the seed logic. Can
> you split that out? (I think these changes are good.)

Ok, two patches that come to mind:

- move string constants solely referenced in __init function to __initconst

- boot_command_line is not guaranteed nul terminated, strlen must be 
replaced with strnlen.

>>   
>>   	if (extra_command_line)
>>   		xlen = strlen(extra_command_line);
> Unrelated note: whoa this is based on linux-next which has a massive
> change to the boot command line handling and appears to be doing some
> bad things. I need to go find that thread...

I took top of linus tree, I did not use linux-next (!) Hopefully all is 
good.

> . . .
>> @@ -875,6 +909,21 @@ asmlinkage __visible void __init start_kernel(void)
>>   	rand_initialize();
>>   	add_latent_entropy();
>>   	add_device_randomness(command_line, strlen(command_line));
>> +	if (IS_BUILTIN(CONFIG_RANDOM_TRUST_BOOTLOADER)) {
>> +		size_t l = strlen(command_line);
>> +		char *rng_seed = strnstr(command_line, rng_seed_str, l);
>> +
>> +		if (rng_seed) {
>> +			char *end;
>> +
>> +			rng_seed += strlen(rng_seed_str);
>> +			l -= rng_seed - command_line;
>> +			end = strnchr(rng_seed, l, ' ');
>> +			if (end)
>> +				l = end - rng_seed;
>> +			credit_trusted_entropy(l);
>> +		}
>> +	}
> Can you pull this out of line and write a new static help that does all
> of the rng stuff here? Basically from rand_initialize() through
> boot_init_stack_canary(), so it's all in one place and not "open coded"
> in start_kernel(). (And then, actually, you don't need a separate
> credit_trusted_entropy() function at all -- just call
> credit_entropy_bits() directly there (and add a comment about the
> command line already getting added).

sgtm, will do.

Thanks both -- Mark