Re: BUG: KCSAN: data-race in add_device_randomness+0x20d/0x290

[Jason A. Donenfeld]

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