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

[Måns Rullgård]

Michael Niedermayer <michaelni at gmx.at> writes:

> On Wed, Jun 30, 2010 at 10:20:29PM +0100, M?ns Rullg?rd wrote:
>> Michael Niedermayer <michaelni at gmx.at> writes:
>> 
>> > 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
>> 
>> You're trying really hard to avoid it...  My question must be answered
>> whatever the answer to yours is.  The converse is not true.
>
> Nothing must be awnsered and there is no such relation between the questions
> as you pretend there is.
>
> Anyway, if nothing is available to generate a random number then the only
> sane thing to do is to return 0
> it is certainly better than returning uninitialized memory.

You _are_ stubborn.  There are systems, many in fact, without
/dev/[u]random or AV_READ_TIME.  Any solution good enough for them is
also good enough for a blocking /dev/random.

-- 
M?ns Rullg?rd
mans at mansr.com