linux-kernel - Re: [PATCH] random: do not use jump labels before they are initialized

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Message-ID: <Yp8wz2Ey4J4u+ZlK@zx2c4.com>
Date:   Tue, 7 Jun 2022 13:04:47 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Stephen Boyd <swboyd@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Phil Elwell <phil@...pberrypi.com>
Subject: Re: [PATCH] random: do not use jump labels before they are
 initialized

Hi Ard,

On Tue, Jun 07, 2022 at 12:56:20PM +0200, Ard Biesheuvel wrote:
> Could we do this to defer the static key manipulation? That way, the
> first call to crng_reseed() that occurs after the static keys API
> becomes available will set the static key, and patch itself away at
> the same time.

That's almost the same as the patch I just posted, except you
pushed the logic down into crng_reseed() instead of credit_init_bits().
(A previous mini-project aimed to remove as much logic as possible from
crng_reseed(), counting on those blocks in crng_init_bits() to only ever
run once.) What this means is that the static key won't get changed
until whenever the next reseeding is. I guess that's "fine" but I think
I'd prefer to keep the entropy counting stuff as separate from the init
bits stuff as possible.

>> As a third, I could just defer doing anything with the bootloader seed
>> until random_init(). This might actually be the simplest solution...
>> I'll sketch something out. A downside, which might be sort of
>> significant, is that a few odd things actually use randomness before
>> random_init() is called. So these would miss out on having that seed.
>> I'll have to look what exactly to see if we're actually getting anything
>> real out of that.
>>
>
> This is kind of the point of using a firmware provided seed, i.e.,
> that it is available much earlier than anything else.

I'll send a patch for this anyway because I'm sort of curious now. Maybe
it'll be a dead end, for the reason you mentioned, but I think I'll
still try to evaluate it.

Jason