linux-kernel - Re: [PATCH 1/2] random: fix locking in crng_fast

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Message-ID: <CAHmME9rsDkn=qRacYbZFjni5FF6G2dn+KOELr-agPhmH7ved0g@mail.gmail.com>
Date:   Sat, 5 Feb 2022 14:17:15 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH 1/2] random: fix locking in crng_fast_load()

Hi Dominik,

Does this introduce a lock nesting inversion situation?

With your patch, crng_fast_load() now does:

    lock(primary_crng)
    invalidate_batched_entropy()
        lock(batch_lock)
        unlock(batch_lock)
    unlock(primary_crng)

While get_random_{u32,u64}() does:

    lock(batch_lock)
    extract_crng()
       lock(primary_crng)
       unlock(primary_crng)
    unlock(batch_lock)

Is this correct? If so, we might have to defer this patch until after
<https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git/commit/?id=2dfab1b1>
or something like it lands, which attempts to get rid of the batched
entropy lock.

If that analysis seems right to you, I could pull this patch into that
development branch for poking and prodding.

Jason