linux-kernel - Re: [PATCH] random32: avoid attempt to late reseed if in the middle of seeding

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Date:	Thu, 27 Mar 2014 10:04:03 +0100
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Sasha Levin <sasha.levin@...cle.com>, davem@...emloft.net,
	tytso@....edu, linux-kernel@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH] random32: avoid attempt to late reseed if in the middle
 of seeding

On 03/27/2014 03:21 AM, Hannes Frederic Sowa wrote:
> On Wed, Mar 26, 2014 at 07:35:01PM -0400, Sasha Levin wrote:
>> On 03/26/2014 07:18 PM, Daniel Borkmann wrote:
>>> On 03/26/2014 06:12 PM, Sasha Levin wrote:
>>>> Commit 4af712e8df ("random32: add prandom_reseed_late() and call when
>>>> nonblocking pool becomes initialized") has added a late reseed stage
>>>> that happens as soon as the nonblocking pool is marked as initialized.
>>>>
>>>> This fails in the case that the nonblocking pool gets initialized
>>>> during __prandom_reseed()'s call to get_random_bytes(). In that case
>>>> we'd double back into __prandom_reseed() in an attempt to do a late
>>>> reseed - deadlocking on 'lock' early on in the boot process.
>>>>
>>>> Instead, just avoid even waiting to do a reseed if a reseed is already
>>>> occuring.
>>>>
>>>> Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
>>>
>>> Thanks for catching! (If you want Dave to pick it up, please also
>>> Cc netdev.)
>>>
>>> Why not via spin_trylock_irqsave() ? Thus, if we already hold the
>>> lock, we do not bother any longer with doing the same work twice
>>> and just return.
>
> I totally agree with Daniel spin_trylock_irqsave seems like the best
> solution.
>
> In case we really want to make sure that even early seeding doesn't
> race with late seed and the pool is only filled by another CPU, we would
> actually need per-cpu bools to get this case correct.

But then again, we would just exit via spin_trylock_irqsave()
now, no? Whenever something enters this section protected under
irq save spinlock we would do a reseed of the entire state (s1-s4)
for each cpu.

>> Your code looks much better, I'll should really stop sending patches
>> too early in the morning...
>>
>> It's also worth adding lib/random32.c to the MAINTAINERS file, as my
>> list of recipients is solely based on what get_maintainer.pl tells
>> me to do (and I'm assuming that I'm not the last person who will be
>> sending patches for this).
>
> Would be a nice idea, especially because prandom_u32 changes are sensitive to
> network security and should get reviewed there, too.

Indeed, sounds good to me.

> Greetings,
>
>    Hannes
>
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