Messages in this thread | | | From | Jann Horn <> | Date | Mon, 7 Feb 2022 22:50:12 +0100 | Subject | Re: BUG: KCSAN: data-race in add_device_randomness+0x20d/0x290 |
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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?
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