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

[Daniel Borkmann]

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@xxxxxxxxxx>
> > >
> > > 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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