Messages in this thread | | | From | "Jason A. Donenfeld" <> | Date | Mon, 3 Jan 2022 17:42:54 +0100 | Subject | Re: [PATCH] random: Don't reset crng_init_cnt on urandom_read() |
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On Mon, Jan 3, 2022 at 5:39 PM Theodore Ts'o <tytso@mit.edu> wrote: > > On Mon, Jan 03, 2022 at 05:03:57PM +0100, Jason A. Donenfeld wrote: > > On Mon, Jan 3, 2022 at 4:59 PM Jann Horn <jannh@google.com> wrote: > > > This code was inconsistent, and it probably made things worse - just get > > > rid of it. > > > > Rather than adding crng_init_cnt=0 if crng_init<1 to extract_crng_user > > and get_random_bytes, getting rid of it like this seems probably okay > > and makes the model simpler. I'll apply this. Thank you. > > Ack. It does mean that we're making a choice that an attacker who is > carrying out a incremental state tracking attack on the CRNG will make > /dev/urandom (and getrandom) to make the crng emit "less secure" in > the period when crng_init is > 0 and < 2. On the other hand, this > allows us to get to the state of crng_init=2 faster, where as before, > the attacker could delay getting us to the state crng_init=1 forever, > where reads from /dev/urandom would be hence be insecure forever (and > getrandom() would block forever).
Right. I had a few early drafts of this commit where I was trying to protect the 0->1 transition from being bruteforced with a trickle of entropy, and Jann's offline comment was something along the lines of, "why do we actually care about crng_init==1? it's not secure anyway," which seems compelling. Plus, as you point out, letting anything reset crng_init_cnt (like /dev/urandom reads) means unprivileged userspace can delay crng_init==2, which seems like a bigger deal.
Jason
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