From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Guenter Roeck <linux@roeck-us.net>
Cc: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
linux-arch@vger.kernel.org, Dinh Nguyen <dinguyen@kernel.org>,
Nick Hu <nickhu@andestech.com>, Max Filippov <jcmvbkbc@gmail.com>,
Palmer Dabbelt <palmer@dabbelt.com>,
"David S . Miller" <davem@davemloft.net>,
Yoshinori Sato <ysato@users.sourceforge.jp>,
Michal Simek <monstr@monstr.eu>, Borislav Petkov <bp@alien8.de>,
Guo Ren <guoren@kernel.org>,
Geert Uytterhoeven <geert@linux-m68k.org>,
Joshua Kinard <kumba@gentoo.org>,
David Laight <David.Laight@aculab.com>,
Dominik Brodowski <linux@dominikbrodowski.net>,
Eric Biggers <ebiggers@google.com>,
Ard Biesheuvel <ardb@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
Thomas Gleixner <tglx@linutronix.de>,
Andy Lutomirski <luto@kernel.org>,
Kees Cook <keescook@chromium.org>,
Lennart Poettering <mzxreary@0pointer.de>,
Konstantin Ryabitsev <konstantin@linuxfoundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH v1] random: block in /dev/urandom
Date: Tue, 22 Mar 2022 12:19:16 -0600 [thread overview]
Message-ID: <YjoTJFRook+rGyDI@zx2c4.com> (raw)
In-Reply-To: <d5c23f68-30ba-a5eb-6bea-501736e79c88@roeck-us.net>
Hi Guenter,
On Tue, Mar 22, 2022 at 10:56:44AM -0700, Guenter Roeck wrote:
> Everything - including the various root file systems - is at
> git@github.com:groeck/linux-build-test.git. Look into rootfs/ for the
> various boot tests. I'll be happy to provide some qemu command lines
> if needed.
Thanks. It looks like the "problem" is with this shell script:
init_rng() {
if check_file_size; then
printf 'Initializing random number generator: '
dd if="$URANDOM_SEED" bs="$pool_size" of=/dev/urandom count=1 2> /dev/null
status=$?
if [ "$status" -eq 0 ]; then
echo "OK"
else
echo "FAIL"
fi
return "$status"
fi
}
save_random_seed() {
printf 'Saving random seed: '
if touch "$URANDOM_SEED" 2> /dev/null; then
old_umask=$(umask)
umask 077
dd if=/dev/urandom of="$URANDOM_SEED" bs="$pool_size" count=1 2> /dev/null
status=$?
umask "$old_umask"
if [ "$status" -eq 0 ]; then
echo "OK"
else
echo "FAIL"
fi
else
status=$?
echo "SKIP (read-only file system detected)"
fi
return "$status"
}
case "$1" in
start|restart|reload)
# Carry a random seed from start-up to start-up
# Load and then save the whole entropy pool
init_rng && save_random_seed;;
This code is actually problematic for a number of reasons. (And Linus,
I'm not saying "userspace is wrong" to justify breaking it or something,
don't worry.)
The first `dd if="$URANDOM_SEED" bs="$pool_size" of=/dev/urandom count=1`
will write the seed into the input pool, but:
- It won't credit the entropy from that seed, so the pool won't
actually initialize. (You need to use the ioctl to credit it.)
- Because the pool doesn't initialize, subsequent reads from
/dev/urandom won't actually use that seed.
The first point is why we had to revert this patch. But the second one
is actually a bit dangerous: you might write in a perfectly good seed to
/dev/urandom, but what you read out for the subsequent seed may be
complete deterministic crap. This is because the call to write_pool()
goes right into the input pool and doesn't touch any of the "fast init"
stuff, where we immediately mutate the crng key during early boot.
As far as I can tell, this has been the behavior for a really long time,
making the above innocuous pattern a pretty old thing that's broken. So
I could perhaps say, "this behavior is so old now, that your userspace
code is just plain broken," but I think I might actually have a very
quick unobtrusive fix for this. I'll mull some things over for rc2 or
later in rc1.
But, anyway, this only fixes the second point mentioned above. The first
one -- which resulted in the revert -- remains a stumper for now.
Jason
next prev parent reply other threads:[~2022-03-22 18:19 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-17 16:28 [PATCH v1] random: block in /dev/urandom Jason A. Donenfeld
2022-02-21 18:01 ` Andy Lutomirski
2022-02-23 17:02 ` Jason A. Donenfeld
2022-02-23 17:55 ` Theodore Ts'o
2022-03-12 20:17 ` Eric Biggers
2022-03-12 20:27 ` Eric Biggers
2022-03-22 15:58 ` Guenter Roeck
2022-03-22 16:21 ` Linus Torvalds
2022-03-22 16:40 ` Jason A. Donenfeld
2022-03-22 17:09 ` Jason A. Donenfeld
2022-03-22 17:56 ` Guenter Roeck
2022-03-22 18:19 ` Jason A. Donenfeld [this message]
2022-03-22 18:29 ` Linus Torvalds
2022-03-22 18:36 ` Jason A. Donenfeld
2022-04-22 13:42 ` Jason A. Donenfeld
2022-04-22 23:46 ` Guenter Roeck
2022-04-23 13:56 ` Guenter Roeck
2022-04-23 14:28 ` Jason A. Donenfeld
2022-04-23 16:35 ` Guenter Roeck
2022-04-23 21:10 ` Jason A. Donenfeld
2022-04-24 2:04 ` Guenter Roeck
2022-04-25 0:12 ` Jason A. Donenfeld
2022-04-25 1:54 ` Guenter Roeck
2022-04-25 11:11 ` Jason A. Donenfeld
2022-03-22 18:24 ` Mark Brown
2022-03-22 21:54 ` Guenter Roeck
2022-03-22 22:25 ` David Laight
2022-03-23 12:10 ` Mark Brown
2022-03-23 14:23 ` Guenter Roeck
2022-03-23 15:53 ` Arnd Bergmann
2022-03-23 16:18 ` Mark Brown
2022-03-23 16:41 ` Arnd Bergmann
2022-03-23 16:47 ` Mark Brown
2022-04-23 0:52 ` Jason A. Donenfeld
2022-04-25 12:09 ` Mark Brown
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