Re: [PATCH v3] random: absorb fast pool into input pool after fast load

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Feb 15, 2022 at 10:13:33PM +0100, Jason A. Donenfeld wrote:
> During crng_init == 0, we never credit entropy in add_interrupt_
> randomness(), but instead dump it directly into the primary_crng. That's
> fine, except for the fact that we then wind up throwing away that
> entropy later when we switch to extracting from the input pool and
> overwriting the primary_crng key. The two other early init sites --
> add_hwgenerator_randomness()'s use crng_fast_load() and add_device_
> randomness()'s use of crng_slow_load() -- always additionally give their
> inputs to the input pool. But not add_interrupt_randomness().
> 
> This commit fixes that shortcoming by calling mix_pool_bytes() after
> crng_fast_load() in add_interrupt_randomness(). That's partially
> verboten on PREEMPT_RT, where it implies taking spinlock_t from an IRQ
> handler. But this also only happens during early boot and then never
> again after that. Plus it's a trylock so it has the same considerations
> as calling crng_fast_load(), which we're already using.
> 
> Cc: Theodore Ts'o <tytso@mit.edu>
> Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
> Suggested-by: Eric Biggers <ebiggers@google.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> v3 uses a trylock instead of a spinlock, just like all the other locks
> taken in hard irq. (Incidentally, we're now talking about moving this
> into the deferred stage, so that at can be a spinlock, but at least with
> what we have here, this really must be a trylock.)

This looks fine, though it's unfortunate that it has to be a trylock so this
isn't guaranteed.  Also, the commit message is a bit misleading because it talks
about "overwriting" the primary_crng key, but at this point in the series the
extracted entropy is still being XOR'd with the primary_crng key.  It's not
until the next patch that the key is simply overwritten.

- Eric