linux-kernel - Re: Linux 5.3-rc8

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Date:   Sat, 14 Sep 2019 17:02:06 +0200
From:   "Ahmed S. Darwish" <darwish.07@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.cz>, Ray Strode <rstrode@...hat.com>,
        William Jon McCann <mccann@....edu>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        zhangjs <zachary@...shancloud.com>, linux-ext4@...r.kernel.org,
        Lennart Poettering <lennart@...ttering.net>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Linux 5.3-rc8

On Thu, Sep 12, 2019 at 12:34:45PM +0100, Linus Torvalds wrote:
> On Thu, Sep 12, 2019 at 9:25 AM Theodore Y. Ts'o <tytso@....edu> wrote:
> >
> > Hmm, one thought might be GRND_FAILSAFE, which will wait up to two
> > minutes before returning "best efforts" randomness and issuing a huge
> > massive warning if it is triggered?
> 
> Yeah, based on (by now) _years_ of experience with people mis-using
> "get me random numbers", I think the sense of a new flag needs to be
> "yeah, I'm willing to wait for it".
>
> Because most people just don't want to wait for it, and most people
> don't think about it, and we need to make the default be for that
> "don't think about it" crowd, with the people who ask for randomness
> sources for a secure key having to very clearly and very explicitly
> say "Yes, I understand that this can take minutes and can only be done
> long after boot".
> 
> Even then people will screw that up because they copy code, or some
> less than gifted rodent writes a library and decides "my library is so
> important that I need that waiting sooper-sekrit-secure random
> number", and then people use that broken library by mistake without
> realizing that it's not going to be reliable at boot time.
> 
> An alternative might be to make getrandom() just return an error
> instead of waiting. Sure, fill the buffer with "as random as we can"
> stuff, but then return -EINVAL because you called us too early.
>

ACK, that's probably _the_ most sensible approach. Only caveat is
the slight change in user-space API semantics though...

For example, this breaks the just released systemd-random-seed(8)
as it _explicitly_ requests blocking behvior from getrandom() here:

    => src/random-seed/random-seed.c:
    /*
     * Let's make this whole job asynchronous, i.e. let's make
     * ourselves a barrier for proper initialization of the
     * random pool.
     */
     k = getrandom(buf, buf_size, GRND_NONBLOCK);
     if (k < 0 && errno == EAGAIN && synchronous) {
         log_notice("Kernel entropy pool is not initialized yet, "
                    "waiting until it is.");
                    
         k = getrandom(buf, buf_size, 0); /* retry synchronously */
     }
     if (k < 0) {
         log_debug_errno(errno, "Failed to read random data with "
                         "getrandom(), falling back to "
                         "/dev/urandom: %m");
     } else if ((size_t) k < buf_size) {
         log_debug("Short read from getrandom(), falling back to "
	           "/dev/urandom: %m");
     } else {
         getrandom_worked = true;
     }

Nonetheless, a slightly broken systemd-random-seed, that was just
released only 11 days ago (v243), is honestly much better than a
*non-booting system*...

I've sent an RFC patch at [1].

To handle the systemd case, I'll add the discussed "yeah, I'm
willing to wait for it" flag (GRND_BLOCK) in v2.

If this whole approach is going to be merged, and the slight ABI
breakage is to be tolerated (hmmmmm?), I wonder how will systemd
random-seed handle the semantics change though without doing
ugly kernel version checks..

thanks,

[1] https://lkml.kernel.org/r/20190914122500.GA1425@darwi-home-pc

--
darwi
http://darwish.chasingpointers.com