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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Mon, 7 Feb 2022 19:42:40 +0100
Message-ID: <CAHmME9pktmNpcBS_DJhJ5Z+6xO9P1wroQ9_gwx8KZMBxk1FBeQ@mail.gmail.com>
Subject: Re: BUG: KCSAN: data-race in add_device_randomness+0x20d/0x290
To:     pmenzel@molgen.mpg.de
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        LKML <linux-kernel@vger.kernel.org>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Jann Horn <jannh@google.com>
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Hi Paul,

Thanks for the report. I assume that this is actually an old bug. Do
you have a vmlinux or a random.o from this kernel you could send me to
double check? Without that, my best guess, which I'd say I have
relatively high confidence about, is that the "1 byte read" is
actually a `movzx eax, cs:lfsr` referring to the `static u8 lfsr`
here, which gets inlined into add_device_randomness:

static int crng_slow_load(const u8 *cp, size_t len)
{
    unsigned long flags;
    static u8 lfsr = 1;

This was added in 2008 with dc12baacb95f ("random: use a different
mixing algorithm for add_device_randomness()"). My understanding is
that the race here isn't super problematic as we're in kind of a half
assed "low effort" phase anyway. But I'll give it some thought. I'm
CCing Jann as well who reported the original issue that motivated that
change.

Regards,
Jason