Linux-ext4 Archive on lore.kernel.org
 help / color / Atom feed
From: "Ahmed S. Darwish" <darwish.07@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lennart Poettering <mzxreary@0pointer.de>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"Alexander E. Patrakov" <patrakov@gmail.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Willy Tarreau <w@1wt.eu>, Matthew Garrett <mjg59@srcf.ucam.org>,
	lkml <linux-kernel@vger.kernel.org>,
	linux-ext4@vger.kernel.org, linux-api@vger.kernel.org,
	linux-man@vger.kernel.org
Subject: Re: [PATCH RFC v4 1/1] random: WARN on large getrandom() waits and introduce getrandom2()
Date: Fri, 20 Sep 2019 15:46:09 +0200
Message-ID: <20190920134609.GA2113@pc> (raw)
In-Reply-To: <CAHk-=wiCqDiU7SE3FLn2W26MS_voUAuqj5XFa1V_tiGTrrW-zQ@mail.gmail.com>

Hi,

On Wed, Sep 18, 2019 at 04:57:58PM -0700, Linus Torvalds wrote:
> On Wed, Sep 18, 2019 at 2:17 PM Ahmed S. Darwish <darwish.07@gmail.com> wrote:
> >
> > Since Linux v3.17, getrandom(2) has been created as a new and more
> > secure interface for pseudorandom data requests.  It attempted to
> > solve three problems, as compared to /dev/urandom:
  > 
> I don't think your patch is really _wrong_, but I think it's silly to
> introduce a new system call, when we have 30 bits left in the flags of
> the old one, and the old system call checked them.
> 
> So it's much simpler and more straightforward to  just introduce a
> single new bit #2 that says "I actually know what I'm doing, and I'm
> explicitly asking for secure/insecure random data".
> 
> And then say that the existing bit #1 just means "I want to wait for entropy".
> 
> So then you end up with this:
> 
>     /*
>      * Flags for getrandom(2)
>      *
>      * GRND_NONBLOCK    Don't block and return EAGAIN instead
>      * GRND_WAIT_ENTROPY        Explicitly wait for entropy
>      * GRND_EXPLICIT    Make it clear you know what you are doing
>      */
>     #define GRND_NONBLOCK               0x0001
>     #define GRND_WAIT_ENTROPY   0x0002
>     #define GRND_EXPLICIT               0x0004
> 
>     #define GRND_SECURE (GRND_EXPLICIT | GRND_WAIT_ENTROPY)
>     #define GRND_INSECURE       (GRND_EXPLICIT | GRND_NONBLOCK)
> 
>     /* Nobody wants /dev/random behavior, nobody should use it */
>     #define GRND_RANDOM 0x0002
> 
> which is actually fairly easy to understand. So now we have three
> bits, and the values are:
> 
>  000  - ambiguous "secure or just lazy/ignorant"
>  001 - -EAGAIN or secure
>  010 - blocking /dev/random DO NOT USE
>  011 - nonblocking /dev/random DO NOT USE
>  100 - nonsense, returns -EINVAL
>  101 - /dev/urandom without warnings
>  110 - blocking secure
>  111 - -EAGAIN or secure
>

Hmmm, the point of the new syscall was **exactly** to avoid the 2^3
combinations above, and to provide developers only two, sane and easy,
options:

  - GRND2_INSECURE
  - GRND2_SECURE_UNBOUNDED_INITIAL_WAIT

You *must* pick one of these, and that's it. (!)

Then the proposed getrandom_wait(7) manpage, also mentioned in the V4
patch WARN message, would provide a big rationale, and encourage
everyone to use the new getrandom2(2) syscall instead.

But yeah, maybe we should add the extra flags to the old getrandom()
instead, and let glibc implement a getrandom_safe(3) wrapper only
with the sane options available.

Problem is, glibc is still *really* slow in adopting linux syscall
wrappers, so I'm not optimistic about that...

I still see the new system call as the sanest path, even provided
the cost of a new syscall number..

@Linus, @Ted:  Final thoughts?

> and people would be encouraged to use one of these three:
> 
>  - GRND_INSECURE
>  - GRND_SECURE
>  - GRND_SECURE | GRND_NONBLOCK
> 
> all of which actually make sense, and none of which have any
> ambiguity. And while "GRND_INSECURE | GRND_NONBLOCK" works, it's
> exactly the same as just plain GRND_INSECURE - the point is that it
> doesn't block for entropy anyway, so non-blocking makes no different.
>

[...]

> 
> There is *one* other small semantic change: The old code did
> urandom_read() which added warnings, but each warning also _reset_ the
> crng_init_cnt. Until it decided not to warn any more, at which point
> it also stops that resetting of crng_init_cnt.
> 
> And that reset of crng_init_cnt, btw, is some cray cray.
> 
> It's basically a "we used up entropy" thing, which is very
> questionable to begin with as the whole discussion has shown, but
> since it stops doing it after 10 cases, it's not even good security
> assuming the "use up entropy" case makes sense in the first place.
> 
> So I didn't copy that insanity either. And I'm wondering if removing
> it from /dev/urandom might also end up helping Ahmed's case of getting
> entropy earlier, when we don't reset the counter.
>

Yeah, noticed that, but I've learned not to change crypto or
speculative-execution code even if the changes "just look the same" at
first glance ;-)

(out of curiosity, I'll do a quick test with this CRNG entropy reset
part removed. Maybe it was indeed part of the problem..)

> But other than those two details, none of the existing semantics
> changed, we just added the three actually _sane_ cases without any
> ambiguity.
> 
> In particular, this still leaves the semantics of that nasty
> "getrandom(0)" as the same "blocking urandom" that it currently is.
> But now it's a separate case, and we can make that perhaps do the
> timeout, or at least the warning.
>

Yeah, I would propose to keep the V4-submitted "timeout then WARN"
logic. This alone will give user-space / distributions time to adapt.

For example, it was interesting that even the 0day bot had limited
entropy on boot (virtio-rng / TRUST_CPU not enabled):

    https://lkml.kernel.org/r/20190920005120.GP15734@shao2-debian

If user-space didn't get its act together, then the other extreme
measures can be implemented later (the getrandom() length test, using
jitter as a credited kernel entropy source, etc., etc.)

> And the new cases are defined to *not* warn. In particular,
> GRND_INSECURE very much does *not* warn about early urandom access
> when crng isn't ready. Because the whole point of that new mode is
> that the user knows it isn't secure.
>
> So that should make getrandom(GRND_INSECURE) palatable to the systemd
> kind of use that wanted to avoid the pointless kernel warning.
>

Yup, that's what was in the submitted V4 patch too. The caller
explicitly asked for "insecure", so they know what they're doing.

getrandom2(2) never prints any kernel message.

> And we could mark this for stable and try to get it backported so that
> it will have better coverage, and encourage people to use the new sane
> _explicit_ waiting (or not) for entropy.
>

ACK. I'll wait for an answer to the "Final thoughts?" question above,
send a V5 with CC:stable, then disappear from this thread ;-)

Thanks a lot everyone!

--
Ahmed Darwish

  parent reply index

Thread overview: 211+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CAHk-=whBQ+6c-h+htiv6pp8ndtv97+45AH9WvdZougDRM6M4VQ@mail.gmail.com>
2019-09-10  4:21 ` Linux 5.3-rc8 Ahmed S. Darwish
2019-09-10 11:33   ` Linus Torvalds
2019-09-10 12:21     ` Linus Torvalds
2019-09-10 17:33     ` Ahmed S. Darwish
2019-09-10 17:47       ` Reindl Harald
2019-09-10 18:21       ` Linus Torvalds
2019-09-11 16:07         ` Theodore Y. Ts'o
2019-09-11 16:45           ` Linus Torvalds
2019-09-11 17:00             ` Linus Torvalds
2019-09-11 17:36               ` Theodore Y. Ts'o
2019-09-12  3:44                 ` Ahmed S. Darwish
2019-09-12  8:25                   ` Theodore Y. Ts'o
2019-09-12 11:34                     ` Linus Torvalds
2019-09-12 11:58                       ` Willy Tarreau
2019-09-14 12:25                       ` [PATCH RFC] random: getrandom(2): don't block on non-initialized entropy pool Ahmed S. Darwish
2019-09-14 14:08                         ` Alexander E. Patrakov
2019-09-15  5:22                           ` [PATCH RFC v2] random: optionally block in getrandom(2) when the CRNG is uninitialized Theodore Y. Ts'o
2019-09-15  8:17                             ` [PATCH RFC v3] random: getrandom(2): optionally block when " Ahmed S. Darwish
2019-09-15  8:59                               ` Lennart Poettering
2019-09-15  9:30                                 ` Willy Tarreau
2019-09-15 10:02                                   ` Ahmed S. Darwish
2019-09-15 10:40                                     ` Willy Tarreau
2019-09-15 10:55                                       ` Ahmed S. Darwish
2019-09-15 11:17                                         ` Willy Tarreau
2019-09-15 17:32                             ` [PATCH RFC v2] random: optionally block in getrandom(2) when the " Linus Torvalds
2019-09-15 18:32                               ` Willy Tarreau
2019-09-15 18:36                                 ` Willy Tarreau
2019-09-15 19:08                                   ` Linus Torvalds
2019-09-15 19:18                                     ` Willy Tarreau
2019-09-15 19:31                                       ` Linus Torvalds
2019-09-15 19:54                                         ` Willy Tarreau
2019-09-15 18:59                                 ` Linus Torvalds
2019-09-15 19:12                                   ` Willy Tarreau
2019-09-16  2:45                                   ` Ahmed S. Darwish
2019-09-16 18:08                               ` Lennart Poettering
2019-09-16 19:16                                 ` Willy Tarreau
2019-09-18 21:15                               ` [PATCH RFC v4 0/1] random: WARN on large getrandom() waits and introduce getrandom2() Ahmed S. Darwish
2019-09-18 21:17                                 ` [PATCH RFC v4 1/1] " Ahmed S. Darwish
2019-09-18 23:57                                   ` Linus Torvalds
2019-09-19 14:34                                     ` Theodore Y. Ts'o
2019-09-19 15:20                                       ` Linus Torvalds
2019-09-19 15:50                                         ` Linus Torvalds
2019-09-20 13:13                                           ` Theodore Y. Ts'o
2019-09-19 20:04                                         ` Linus Torvalds
2019-09-19 20:45                                           ` Alexander E. Patrakov
2019-09-19 21:47                                             ` Linus Torvalds
2019-09-19 22:23                                               ` Alexander E. Patrakov
2019-09-19 23:44                                                 ` Alexander E. Patrakov
2019-09-20 13:16                                                 ` Theodore Y. Ts'o
2019-09-23 11:55                                           ` David Laight
2019-09-20 13:08                                         ` Theodore Y. Ts'o
2019-09-20 13:46                                     ` Ahmed S. Darwish [this message]
2019-09-20 14:33                                       ` Andy Lutomirski
2019-09-20 16:29                                         ` Linus Torvalds
2019-09-20 17:52                                           ` Andy Lutomirski
2019-09-20 18:09                                             ` Linus Torvalds
2019-09-20 18:16                                               ` Willy Tarreau
2019-09-20 19:12                                               ` Andy Lutomirski
2019-09-20 19:51                                                 ` Linus Torvalds
2019-09-20 20:11                                                   ` Alexander E. Patrakov
2019-09-20 20:17                                                   ` Matthew Garrett
2019-09-20 20:51                                                   ` Andy Lutomirski
2019-09-20 22:44                                                     ` Linus Torvalds
2019-09-20 23:30                                                       ` Andy Lutomirski
2019-09-21  3:05                                                         ` Willy Tarreau
2019-09-21  6:07                                               ` Florian Weimer
2019-09-23 18:33                                                 ` Andy Lutomirski
2019-09-26 21:11                                                   ` Ahmed S. Darwish
2019-09-20 18:12                                             ` Willy Tarreau
2019-09-20 19:22                                               ` Andy Lutomirski
2019-09-20 19:37                                                 ` Willy Tarreau
2019-09-20 19:52                                                   ` Andy Lutomirski
2019-09-20 20:02                                                 ` Linus Torvalds
2019-09-20 18:15                                             ` Alexander E. Patrakov
2019-09-20 18:29                                               ` Andy Lutomirski
2019-09-20 17:26                                       ` Willy Tarreau
2019-09-20 17:56                                         ` Ahmed S. Darwish
2019-09-26 20:42                                     ` [PATCH v5 0/1] random: getrandom(2): warn on large CRNG waits, introduce new flags Ahmed S. Darwish
2019-09-26 20:44                                       ` [PATCH v5 1/1] " Ahmed S. Darwish
2019-09-26 21:39                                         ` Andy Lutomirski
2019-09-28  9:30                                           ` Ahmed S. Darwish
2019-09-14 15:02                       ` Linux 5.3-rc8 Ahmed S. Darwish
2019-09-14 16:30                         ` Linus Torvalds
2019-09-14 16:35                           ` Alexander E. Patrakov
2019-09-14 16:52                             ` Linus Torvalds
2019-09-14 17:09                               ` Alexander E. Patrakov
2019-09-14 19:19                                 ` Linus Torvalds
2019-09-15  6:56                               ` Lennart Poettering
2019-09-15  7:01                                 ` Willy Tarreau
2019-09-15  7:05                                   ` Lennart Poettering
2019-09-15  7:07                                     ` Willy Tarreau
2019-09-15  8:34                                       ` Lennart Poettering
2019-09-15 17:02                                 ` Linus Torvalds
2019-09-16  3:23                                   ` Theodore Y. Ts'o
2019-09-16  3:40                                     ` Linus Torvalds
2019-09-16  3:56                                       ` Linus Torvalds
2019-09-16 17:00                                       ` Theodore Y. Ts'o
2019-09-16 17:07                                         ` Linus Torvalds
2019-09-14 21:11                           ` Ahmed S. Darwish
2019-09-14 22:05                             ` Martin Steigerwald
2019-09-14 22:24                             ` Theodore Y. Ts'o
2019-09-14 22:32                               ` Linus Torvalds
2019-09-15  1:00                                 ` Theodore Y. Ts'o
2019-09-15  1:10                                   ` Linus Torvalds
2019-09-15  2:05                                     ` Theodore Y. Ts'o
2019-09-15  2:11                                       ` Linus Torvalds
2019-09-15  6:33                                       ` Willy Tarreau
2019-09-15  6:53                                       ` Willy Tarreau
2019-09-15  6:51                           ` Lennart Poettering
2019-09-15  7:27                             ` Ahmed S. Darwish
2019-09-15  8:48                               ` Lennart Poettering
2019-09-15 16:29                             ` Linus Torvalds
2019-09-16  1:40                               ` Ahmed S. Darwish
2019-09-16  1:48                                 ` Vito Caputo
2019-09-16  2:49                                   ` Theodore Y. Ts'o
2019-09-16  4:29                                     ` Willy Tarreau
2019-09-16  5:02                                       ` Linus Torvalds
2019-09-16  6:12                                         ` Willy Tarreau
2019-09-16 16:17                                           ` Linus Torvalds
2019-09-16 17:21                                             ` Theodore Y. Ts'o
2019-09-16 17:44                                               ` Linus Torvalds
2019-09-16 17:55                                                 ` Serge Belyshev
2019-09-16 19:08                                                 ` Willy Tarreau
2019-09-16 23:02                                                 ` Matthew Garrett
2019-09-16 23:05                                                   ` Linus Torvalds
2019-09-16 23:11                                                     ` Matthew Garrett
2019-09-16 23:13                                                       ` Alexander E. Patrakov
2019-09-16 23:15                                                         ` Matthew Garrett
2019-09-16 23:18                                                       ` Linus Torvalds
2019-09-16 23:29                                                         ` Ahmed S. Darwish
2019-09-17  1:05                                                           ` Linus Torvalds
2019-09-17  1:23                                                             ` Matthew Garrett
2019-09-17  1:41                                                               ` Linus Torvalds
2019-09-17  1:46                                                                 ` Matthew Garrett
2019-09-17  5:24                                                                   ` Willy Tarreau
2019-09-17  7:33                                                                     ` Martin Steigerwald
2019-09-17  8:35                                                                       ` Willy Tarreau
2019-09-17  8:44                                                                         ` Martin Steigerwald
2019-09-17 12:11                                                                       ` Theodore Y. Ts'o
2019-09-17 12:30                                                                         ` Ahmed S. Darwish
2019-09-17 12:46                                                                           ` Alexander E. Patrakov
2019-09-17 12:47                                                                           ` Willy Tarreau
2019-09-17 16:08                                                                           ` Lennart Poettering
2019-09-17 16:23                                                                             ` Linus Torvalds
2019-09-17 16:34                                                                               ` Reindl Harald
2019-09-17 17:42                                                                               ` Lennart Poettering
2019-09-17 18:01                                                                                 ` Linus Torvalds
2019-09-17 20:28                                                                                   ` Martin Steigerwald
2019-09-17 20:52                                                                                     ` Ahmed S. Darwish
2019-09-17 21:38                                                                                       ` Martin Steigerwald
2019-09-17 21:52                                                                                         ` Matthew Garrett
2019-09-17 22:10                                                                                           ` Martin Steigerwald
2019-09-18 13:53                                                                                             ` Lennart Poettering
2019-09-19  7:28                                                                                               ` Martin Steigerwald
2019-09-17 23:08                                                                                           ` Linus Torvalds
2019-09-18 13:40                                                                                         ` Lennart Poettering
2019-09-17 20:58                                                                                   ` Linus Torvalds
2019-09-18  9:33                                                                                     ` Rasmus Villemoes
2019-09-18 10:16                                                                                       ` Willy Tarreau
2019-09-18 10:25                                                                                         ` Alexander E. Patrakov
2019-09-18 10:42                                                                                           ` Willy Tarreau
2019-09-18 19:31                                                                                       ` Linus Torvalds
2019-09-18 19:56                                                                                 ` Eric W. Biederman
2019-09-18 20:13                                                                                   ` Linus Torvalds
2019-09-18 20:15                                                                                   ` Alexander E. Patrakov
2019-09-18 20:26                                                                                     ` Linus Torvalds
2019-09-18 22:12                                                                                       ` Willy Tarreau
2019-09-27 13:57                                                                                       ` Lennart Poettering
2019-09-27 15:58                                                                                         ` Linus Torvalds
2019-09-29  9:05                                                                                           ` Lennart Poettering
2019-09-17 13:11                                                                         ` Alexander E. Patrakov
2019-09-17 13:37                                                                           ` Alexander E. Patrakov
2019-09-17 15:57                                                                         ` Lennart Poettering
2019-09-17 16:21                                                                           ` Willy Tarreau
2019-09-17 17:13                                                                             ` Lennart Poettering
2019-09-17 17:29                                                                               ` Willy Tarreau
2019-09-17 20:42                                                                                 ` Martin Steigerwald
2019-09-18 13:38                                                                                 ` Lennart Poettering
2019-09-18 13:59                                                                                   ` Alexander E. Patrakov
2019-09-18 14:50                                                                                     ` Alexander E. Patrakov
2019-09-17 20:36                                                                             ` Martin Steigerwald
2019-09-17 16:27                                                                       ` Linus Torvalds
2019-09-17 16:34                                                                         ` Matthew Garrett
2019-09-17 17:16                                                                           ` Willy Tarreau
2019-09-17 17:20                                                                             ` Matthew Garrett
2019-09-17 17:23                                                                               ` Matthew Garrett
2019-09-17 17:57                                                                               ` Willy Tarreau
2019-09-17 16:58                                                                         ` Alexander E. Patrakov
2019-09-17 17:30                                                                           ` Lennart Poettering
2019-09-17 17:32                                                                             ` Willy Tarreau
2019-09-17 17:41                                                                               ` Alexander E. Patrakov
2019-09-17 17:28                                                                         ` Lennart Poettering
2019-09-17  0:03                                                         ` Matthew Garrett
2019-09-17  0:40                                                         ` Matthew Garrett
2019-09-17  7:15                                                     ` a sane approach to random numbers (was: Re: Linux 5.3-rc8) Martin Steigerwald
2019-09-16 18:00                                               ` Linux 5.3-rc8 Alexander E. Patrakov
2019-09-16 19:53                                               ` Ahmed S. Darwish
2019-09-17 15:32                                               ` Lennart Poettering
2019-09-16  3:31                                 ` Linus Torvalds
2019-09-23 20:49                           ` chaos generating driver was " Pavel Machek
2019-09-14  9:25                     ` Ahmed S. Darwish
2019-09-14 16:27                       ` Theodore Y. Ts'o
2019-09-11 21:41             ` Ahmed S. Darwish
2019-09-11 22:37               ` Ahmed S. Darwish
2019-09-16  3:52           ` Herbert Xu
2019-09-16  4:21             ` Linus Torvalds
2019-09-16  4:53               ` Willy Tarreau
2019-09-10 11:56   ` Theodore Y. Ts'o
2019-09-16 10:33     ` Christoph Hellwig
2019-10-03 21:10   ` Jon Masters
2019-10-03 21:31   ` Jon Masters

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190920134609.GA2113@pc \
    --to=darwish.07@gmail.com \
    --cc=ebiederm@xmission.com \
    --cc=linux-api@vger.kernel.org \
    --cc=linux-ext4@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-man@vger.kernel.org \
    --cc=mjg59@srcf.ucam.org \
    --cc=mtk.manpages@gmail.com \
    --cc=mzxreary@0pointer.de \
    --cc=patrakov@gmail.com \
    --cc=torvalds@linux-foundation.org \
    --cc=tytso@mit.edu \
    --cc=w@1wt.eu \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-ext4 Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-ext4/0 linux-ext4/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-ext4 linux-ext4/ https://lore.kernel.org/linux-ext4 \
		linux-ext4@vger.kernel.org
	public-inbox-index linux-ext4

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-ext4


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git