Re: [PATCH 2/3] random: rng-seed source is utf-8

Mark Salyzyn
2020-02-18T16:01:56Z

On 2/14/20 4:53 PM, Theodore Y. Ts'o wrote:
> On Fri, Feb 14, 2020 at 02:55:36PM -0800, Mark Salyzyn wrote:
>>> This is why I really think what gets specified via the boot command
>>> line, or bootconfig, should specify the bits of entropy and the
>>> entropy seed *separately*, so it can be specified explicitly, instead
>>> of assuming that *everyone knows* that rng-seed is either (a) a binary
>>> string, or (b) utf-8, or (c) a hex string. The fact is, everyone does
>>> *not* know, or everyone will have a different implementation, which
>>> everyone will say is *obviously* the only way to go....
>>>
>> Given that the valid option are between 4 (hex), 6 (utf-8) or 8 (binary), we
>> can either split the difference and accept 6; or take a pass at the values
>> and determine which of the set they belong to [0-9a-fA-F], [!-~] or
>> [\000-\377]  nor need to separately specify.
> So let's split this up into separate issues. First of all, from an
> architectural issue, I really think we need to change
> add_bootloader_randomness() in drivers/char/random.c so it looks like this:
>
> void add_bootloader_randomness(const void *buf, unsigned int size, unsigned int entropy_bits)
>
> That's because this is a general function that could be used by any
> number of bootloaders. For example, for the UEFI bootloader, it can
> use the UEFI call that will return binary bits. Some other bootloader
> might use utf-8, etc. So it would be an abstraction violation to have
> code in drivers/char/random.c make assumption about how a particular
> bootloader might be behaving.
>
> The second question is we are going to be parsing an rng_seed
> parameter it shows up in bootconfig or in the boot command line, how
> do we decide how many bits of entropy it actually has. The advantage
> of using the boot command line is we don't need to change the rest of
> the bootloader ecosystem. But that's also a massive weakness, since
> apparently some people are already using it, and perhaps not in the
> same way.
>
> So what I'd really prefer is if we use something new, and we define it
> in a way that makes as close as possible to "impossible to misuse".
> (See Rusty Russell's API design manifesto[1]). So I'm going to
> propose something different. Let's use something new, say
> entropy_seed_hex, and let's say that it *must* be a hex string:
>
> entropy_seed_hex=7337db91a4824e3480ba6d2dfaa60bec
>
> If it is not a valid hex string, it gets zero entropy credit.
>
> I don't think we really need to worry about efficient encoding of the
> seed, since 256 bits is only 64 characters using a hex string. An
> whether it's 32 characters or 64 characters, the max command line
> string is 32k, so it's probably not worth it to try to do something
> more complex. (And only 128 bits is needed to declare the CRNG to be
> fully initialized, in which case we're talking about 16 characters
> versus 32 charaters.)
>
> [1] http://sweng.the-davies.net/Home/rustys-api-design-manifesto
>
> - Ted
>
I am additionally concerned about add_bootloader_randomness() because it
is possible for it to sleep because of add_hwgenerator_randomness() as
once it hits the entropy threshold. As-is it can not be used inside
start_kernel() because the sleep would result in a kernel panic, and I
suspect its use inside early_init_dt_scan_chosen() for the commit "fdt:
add support for rng-seed" might also be problematic since it is
effectively called underneath start_kernel() is it not?

If add_bootloader_randomness was rewritten to call
add_device_randomness() always, and when trusted also called the
functionality of the new credit_trusted_entropy_bits (no longer needing
to be exported if so), then the function could be used in both
start_kernel() and early_init_dt_scan_chosen().

-- Mark