Messages in this thread | | | Subject | Re: [PATCH 2/3] random: rng-seed source is utf-8 | From | Mark Salyzyn <> | Date | Tue, 18 Feb 2020 08:01:51 -0800 |
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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
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