On Tue, Feb 08, 2022 at 04:53:29PM +0100, Jason A. Donenfeld wrote: > When /dev/random was directly connected with entropy extraction, without > any expansion stage, extract_buf() was called for every 10 bytes of data > read from /dev/random. For that reason, RDRAND was used rather than > RDSEED. At the same time, crng_reseed() was still only called every 5 > minutes, so there RDSEED made sense. > > Those olden days were also a time when the entropy collector did not use > a cryptographic hash function, which meant most bets were off in terms > of real preimage resistance. For that reason too it didn't matter > _that_ much whether RDSEED was mixed in before or after entropy > extraction; both choices were sort of bad. > > But now we have a cryptographic hash function at work, and with that we > get real preimage resistance. We also now only call extract_entropy() > every 5 minutes, rather than every 10 bytes. This allows us to do two > important things. > > First, we can switch to using RDSEED in extract_entropy(), as Dominik > suggested. Second, we can ensure that RDSEED input always goes into the > cryptographic hash function with other things before being used > directly. This eliminates a category of attacks in which the CPU knows > the current state of the crng and knows that we're going to xor RDSEED > into it, and so it computes a malicious RDSEED. By going through our > hash function, it would require the CPU to compute a preimage on the > fly, which isn't going to happen. > > Cc: Theodore Ts'o <tytso@xxxxxxx> > Suggested-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx> > --- > drivers/char/random.c | 22 +++++++++------------- > 1 file changed, 9 insertions(+), 13 deletions(-) Looks good, Reviewed-by: Eric Biggers <ebiggers@xxxxxxxxxx> - Eric
