[FFmpeg-devel] [PATCH] Make av_get_random_seed not block when waiting for more entropy

[Michael Niedermayer]

On Wed, Jun 30, 2010 at 09:53:55PM +0100, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > On Wed, Jun 30, 2010 at 09:07:40PM +0100, M?ns Rullg?rd wrote:
> >> Michael Niedermayer <michaelni at gmx.at> writes:
> >> 
> >> > On Wed, Jun 30, 2010 at 07:28:16PM +0300, Martin Storsj? wrote:
> >> >> On Wed, 30 Jun 2010, M?ns Rullg?rd wrote:
> >> >> 
> >> >> > Martin Storsj? <martin at martin.st> writes:
> >> >> > 
> >> >> > > On Wed, 30 Jun 2010, Michael Niedermayer wrote:
> >> >> > >
> >> >> > >> before you spend more time on this.
> >> >> > >> There is a possible security issue with using non block mode
> >> >> > >> namely if we have /dev/*random and not use it we can end up
> >> >> > >> using a uninitialized variable. Thats an information leak
> >> >> > >> it could leak from pointers (kills ASLR) to OS/platform or
> >> >> > >> compiler version or or or ...
> >> >> > >> thats all usefull information for a attacker
> >> >> > >> he only has to saturate /dev/random so it would block
> >> >> > >
> >> >> > > Could you elaborate on your concern here? The fix he committed tries
> >> >> > > both /dev/random and /dev/urandom, and the latter should never block
> >> >> > > (afaik), and even in that case I don't see where any uninitialized
> >> >> > > variable would be leaked?
> >> >> > 
> >> >> > If neither of the files exist, or only /dev/random exists and blocks,
> >> >> > it will return an uninitialised value.  It changes only on systems
> >> >> > that have a blocking /dev/random and no /dev/urandom.
> >> >> 
> >> >> True, except that we'd use AV_READ_TIME in the not all that rare cases 
> >> >> where it is defined. But for that case where it isn't, what about trying 
> >> >> /dev/random in blocking mode only if both the others have failed in 
> >> >> nonblocking mode?
> >> >
> >> > possible
> >> >
> >> > though maybe we could simplify this by asking which combinations actually
> >> > exist?
> >> 
> >> Given that systems without any of the /dev files certainly exist, we
> >> definitely need a sensible fallback for this case.  AV_READ_TIME is
> >> only defined for a few arches.
> >
> > i wonder if systems with a blocking random and no urandom
> > exist because if not we dont need the non blocking code. We can just
> > do blocking reads
> 
> That doesn't address the question of what to do when both of them
> fail to deliver, whether through blocking or nonexistence.

no it doesnt address that question. Thats no reason though not to
adress the other question

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I hate to see young programmers poisoned by the kind of thinking
Ulrich Drepper puts forward since it is simply too narrow -- Roman Shaposhnik
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