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

[Martin Storsjö]

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?

// Martin