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From:   Jann Horn <jannh@google.com>
Date:   Mon, 7 Feb 2022 22:50:12 +0100
Message-ID: <CAG48ez3wZOZZX1UHM-Q=KhOnnGR85Unm08q7jT_wVfOq0PW94Q@mail.gmail.com>
Subject: Re: BUG: KCSAN: data-race in add_device_randomness+0x20d/0x290
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     pmenzel@molgen.mpg.de, "Theodore Y. Ts'o" <tytso@mit.edu>,
        LKML <linux-kernel@vger.kernel.org>,
        Dominik Brodowski <linux@dominikbrodowski.net>
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On Mon, Feb 7, 2022 at 10:45 PM Jann Horn <jannh@google.com> wrote:
> On Mon, Feb 7, 2022 at 7:42 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> > 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.
>
> But the "lfsr" variable is never accessed outside the part of this
> method that holds a global spinlock. So that can't really be it,
> right?

There is a data race in crng_ready(), it just loads from "crng_init"
without READ_ONCE()... maybe that's what KCSAN is noticing?