linux-kernel - Re: BUG: KCSAN: data-race in add_device

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Date:   Mon, 7 Feb 2022 23:20:47 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     pmenzel@...gen.mpg.de, "Theodore Y. Ts'o" <tytso@....edu>,
        LKML <linux-kernel@...r.kernel.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: BUG: KCSAN: data-race in add_device_randomness+0x20d/0x290

Narrowing this a bit by looking at the disassembly from as similar as
a config as I could infer, there are three calls to _tsan_read1():

1. ... = lfsr;
2. ... = dest_buf[i % CHACHA_KEY_SIZE];
3. ... ^= src_buf[i % len] ...

1. lfsr, as you mentioned, appears protected by primary_crng.lock.
2. dest_buf is &primary_crng.state, which /should/ be protected by
primary_crng.lock. The only place where it's not is rand_initialize(),
but this is an early boot function that should complete well in
advance of where we are.
3. That would be tsk->se.sum_exec_runtime...