From: Szabolcs Nagy <szabolcs.nagy@arm.com>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: libc-alpha@sourceware.org, adhemerval.zanella@linaro.org,
Florian Weimer <fweimer@redhat.com>,
Eric Biggers <ebiggers@kernel.org>,
linux-crypto@vger.kernel.org
Subject: Re: [PATCH v6] arc4random: simplify design for better safety
Date: Thu, 28 Jul 2022 11:29:30 +0100 [thread overview]
Message-ID: <YuJlCijOtAXOco6h@arm.com> (raw)
In-Reply-To: <20220726195822.1223048-1-Jason@zx2c4.com>
The 07/26/2022 21:58, Jason A. Donenfeld via Libc-alpha wrote:
> Rather than buffering 16 MiB of entropy in userspace (by way of
> chacha20), simply call getrandom() every time.
>
> This approach is doubtlessly slower, for now, but trying to prematurely
> optimize arc4random appears to be leading toward all sorts of nasty
> properties and gotchas. Instead, this patch takes a much more
> conservative approach. The interface is added as a basic loop wrapper
> around getrandom(), and then later, the kernel and libc together can
> work together on optimizing that.
>
> This prevents numerous issues in which userspace is unaware of when it
> really must throw away its buffer, since we avoid buffering all
> together. Future improvements may include userspace learning more from
> the kernel about when to do that, which might make these sorts of
> chacha20-based optimizations more possible. The current heuristic of 16
> MiB is meaningless garbage that doesn't correspond to anything the
> kernel might know about. So for now, let's just do something
> conservative that we know is correct and won't lead to cryptographic
> issues for users of this function.
>
> This patch might be considered along the lines of, "optimization is the
> root of all evil," in that the much more complex implementation it
> replaces moves too fast without considering security implications,
> whereas the incremental approach done here is a much safer way of going
> about things. Once this lands, we can take our time in optimizing this
> properly using new interplay between the kernel and userspace.
>
> getrandom(0) is used, since that's the one that ensures the bytes
> returned are cryptographically secure. But on systems without it, we
> fallback to using /dev/urandom. This is unfortunate because it means
> opening a file descriptor, but there's not much of a choice. Secondly,
> as part of the fallback, in order to get more or less the same
> properties of getrandom(0), we poll on /dev/random, and if the poll
> succeeds at least once, then we assume the RNG is initialized. This is a
> rough approximation, as the ancient "non-blocking pool" initialized
> after the "blocking pool", not before, and it may not port back to all
> ancient kernels, though it does to all kernels supported by glibc
> (≥3.2), so generally it's the best approximation we can do.
>
> The motivation for including arc4random, in the first place, is to have
> source-level compatibility with existing code. That means this patch
> doesn't attempt to litigate the interface itself. It does, however,
> choose a conservative approach for implementing it.
>
> Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
> Cc: Florian Weimer <fweimer@redhat.com>
> Cc: Cristian Rodríguez <crrodriguez@opensuse.org>
> Cc: Paul Eggert <eggert@cs.ucla.edu>
> Cc: Mark Harris <mark.hsj@gmail.com>
> Cc: Eric Biggers <ebiggers@kernel.org>
> Cc: linux-crypto@vger.kernel.org
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
fyi, after this patch i see
FAIL: stdlib/tst-arc4random-thread
with
$ cat stdlib/tst-arc4random-thread.out
info: arc4random: minimum of 1750000 blob results expected
info: arc4random: 1750777 blob results observed
info: arc4random_buf: minimum of 1750000 blob results expected
info: arc4random_buf: 1750000 blob results observed
info: arc4random_uniform: minimum of 1750000 blob results expected
Timed out: killed the child process
Termination time: 2022-07-27T14:41:33.766791947
Last write to standard output: 2022-07-27T14:41:22.522497854
on an arm and aarch64 builder.
running it manually it takes >30s to complete.
> ---
> LICENSES | 23 -
> NEWS | 4 +-
> include/stdlib.h | 3 -
> manual/math.texi | 13 +-
> stdlib/Makefile | 2 -
> stdlib/arc4random.c | 196 ++----
> stdlib/arc4random.h | 48 --
> stdlib/chacha20.c | 191 ------
> stdlib/tst-arc4random-chacha20.c | 167 -----
> sysdeps/aarch64/Makefile | 4 -
> sysdeps/aarch64/chacha20-aarch64.S | 314 ----------
> sysdeps/aarch64/chacha20_arch.h | 40 --
> sysdeps/generic/chacha20_arch.h | 24 -
> sysdeps/generic/not-cancel.h | 3 +
> sysdeps/generic/tls-internal-struct.h | 1 -
> sysdeps/generic/tls-internal.c | 10 -
> sysdeps/mach/hurd/_Fork.c | 2 -
> sysdeps/mach/hurd/not-cancel.h | 4 +
> sysdeps/nptl/_Fork.c | 2 -
> .../powerpc/powerpc64/be/multiarch/Makefile | 4 -
> .../powerpc64/be/multiarch/chacha20-ppc.c | 1 -
> .../powerpc64/be/multiarch/chacha20_arch.h | 42 --
> sysdeps/powerpc/powerpc64/power8/Makefile | 5 -
> .../powerpc/powerpc64/power8/chacha20-ppc.c | 256 --------
> .../powerpc/powerpc64/power8/chacha20_arch.h | 37 --
> sysdeps/s390/s390-64/Makefile | 6 -
> sysdeps/s390/s390-64/chacha20-s390x.S | 573 ------------------
> sysdeps/s390/s390-64/chacha20_arch.h | 45 --
> sysdeps/unix/sysv/linux/not-cancel.h | 8 +-
> sysdeps/unix/sysv/linux/tls-internal.c | 10 -
> sysdeps/unix/sysv/linux/tls-internal.h | 1 -
> sysdeps/x86_64/Makefile | 7 -
> sysdeps/x86_64/chacha20-amd64-avx2.S | 328 ----------
> sysdeps/x86_64/chacha20-amd64-sse2.S | 311 ----------
> sysdeps/x86_64/chacha20_arch.h | 55 --
> 35 files changed, 64 insertions(+), 2676 deletions(-)
> delete mode 100644 stdlib/arc4random.h
> delete mode 100644 stdlib/chacha20.c
> delete mode 100644 stdlib/tst-arc4random-chacha20.c
> delete mode 100644 sysdeps/aarch64/chacha20-aarch64.S
> delete mode 100644 sysdeps/aarch64/chacha20_arch.h
> delete mode 100644 sysdeps/generic/chacha20_arch.h
> delete mode 100644 sysdeps/powerpc/powerpc64/be/multiarch/Makefile
> delete mode 100644 sysdeps/powerpc/powerpc64/be/multiarch/chacha20-ppc.c
> delete mode 100644 sysdeps/powerpc/powerpc64/be/multiarch/chacha20_arch.h
> delete mode 100644 sysdeps/powerpc/powerpc64/power8/chacha20-ppc.c
> delete mode 100644 sysdeps/powerpc/powerpc64/power8/chacha20_arch.h
> delete mode 100644 sysdeps/s390/s390-64/chacha20-s390x.S
> delete mode 100644 sysdeps/s390/s390-64/chacha20_arch.h
> delete mode 100644 sysdeps/x86_64/chacha20-amd64-avx2.S
> delete mode 100644 sysdeps/x86_64/chacha20-amd64-sse2.S
> delete mode 100644 sysdeps/x86_64/chacha20_arch.h
next prev parent reply other threads:[~2022-07-28 10:30 UTC|newest]
Thread overview: 74+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-23 16:22 arc4random - are you sure we want these? Jason A. Donenfeld
2022-07-23 16:25 ` Jason A. Donenfeld
2022-07-23 17:18 ` Paul Eggert
2022-07-24 23:55 ` Jason A. Donenfeld
2022-07-25 20:31 ` Paul Eggert
2022-07-23 17:39 ` Adhemerval Zanella Netto
2022-07-23 22:54 ` Jason A. Donenfeld
2022-07-25 15:33 ` Rich Felker
2022-07-25 15:59 ` Adhemerval Zanella Netto
2022-07-25 16:18 ` Sandy Harris
2022-07-25 16:40 ` Florian Weimer
2022-07-25 16:51 ` Jason A. Donenfeld
2022-07-25 17:44 ` Rich Felker
2022-07-25 18:33 ` Cristian Rodríguez
2022-07-25 18:49 ` Rich Felker
2022-07-25 18:49 ` Rich Felker
[not found] ` <YuCa1lDqoxdnZut/@mit.edu>
[not found] ` <a5b6307d-6811-61b6-c13d-febaa6ad1e48@linaro.org>
[not found] ` <YuEwR0bJhOvRtmFe@mit.edu>
2022-07-27 12:49 ` Florian Weimer
2022-07-27 20:15 ` Theodore Ts'o
2022-07-27 21:59 ` Rich Felker
2022-07-28 0:30 ` Theodore Ts'o
2022-07-28 0:39 ` Cristian Rodríguez
2022-07-23 19:04 ` Cristian Rodríguez
2022-07-23 22:59 ` Jason A. Donenfeld
2022-07-24 16:23 ` Cristian Rodríguez
2022-07-24 21:57 ` Jason A. Donenfeld
2022-07-25 10:14 ` Florian Weimer
2022-07-25 10:11 ` Florian Weimer
2022-07-25 11:04 ` Jason A. Donenfeld
2022-07-25 12:39 ` Florian Weimer
2022-07-25 13:43 ` Jason A. Donenfeld
2022-07-25 13:58 ` Cristian Rodríguez
2022-07-25 16:06 ` Rich Felker
2022-07-25 16:43 ` Florian Weimer
2022-07-26 14:27 ` Overwrittting AT_RANDOM after use (was Re: arc4random - are you sure we want these?) Yann Droneaud
2022-07-26 14:35 ` arc4random - are you sure we want these? Yann Droneaud
2022-07-25 13:25 ` Jeffrey Walton
2022-07-25 13:48 ` Jason A. Donenfeld
2022-07-25 14:56 ` Rich Felker
2022-07-25 22:57 ` [PATCH] arc4random: simplify design for better safety Jason A. Donenfeld
2022-07-25 23:11 ` Jason A. Donenfeld
2022-07-25 23:28 ` [PATCH v2] " Jason A. Donenfeld
2022-07-25 23:59 ` Eric Biggers
2022-07-26 10:26 ` Jason A. Donenfeld
2022-07-26 1:10 ` Mark Harris
2022-07-26 10:41 ` Jason A. Donenfeld
2022-07-26 11:06 ` Florian Weimer
2022-07-26 16:51 ` Mark Harris
2022-07-26 18:42 ` Jason A. Donenfeld
2022-07-26 19:24 ` Jason A. Donenfeld
2022-07-26 9:55 ` Florian Weimer
2022-07-26 11:04 ` Jason A. Donenfeld
2022-07-26 11:07 ` [PATCH v3] " Jason A. Donenfeld
2022-07-26 11:11 ` Jason A. Donenfeld
2022-07-26 11:12 ` [PATCH v2] " Florian Weimer
2022-07-26 11:20 ` Jason A. Donenfeld
2022-07-26 11:35 ` Adhemerval Zanella Netto
2022-07-26 11:33 ` Adhemerval Zanella Netto
2022-07-26 11:54 ` Jason A. Donenfeld
2022-07-26 12:08 ` Jason A. Donenfeld
2022-07-26 12:20 ` Jason A. Donenfeld
2022-07-26 12:34 ` Adhemerval Zanella Netto
2022-07-26 12:47 ` Jason A. Donenfeld
2022-07-26 13:11 ` Adhemerval Zanella Netto
2022-07-26 13:30 ` [PATCH v4] " Jason A. Donenfeld
2022-07-26 15:21 ` Yann Droneaud
2022-07-26 16:20 ` Adhemerval Zanella Netto
2022-07-26 18:36 ` Jason A. Donenfeld
2022-07-26 19:08 ` [PATCH v5] " Jason A. Donenfeld
2022-07-26 19:58 ` [PATCH v6] " Jason A. Donenfeld
2022-07-26 20:17 ` Adhemerval Zanella Netto
2022-07-26 20:56 ` Adhemerval Zanella Netto
2022-07-28 10:29 ` Szabolcs Nagy [this message]
2022-07-28 10:36 ` Szabolcs Nagy
2022-07-28 11:01 ` Adhemerval Zanella
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