Bug#343085: exim4: Exim SMTP_AUTH hangs since today...

[Sven Hartge]

Um 23:02 Uhr am 30.01.06 schrieb Florian Weimer:
> Sven Hartge:
>> Um 15:15 Uhr am 30.01.06 schrieb Florian Weimer:

>>> Turns out the patch was broken.  This one should be better.  The 
>>> comments above still apply.
>> Sorry, but I patched and recompiled the exim4-package from Sarge, but 
>> any encrypted mail transfer nearly empties the entropy pool.
 
> Again, this is expected -- I tried to fix the blocking problem, not the 
> entropy consumption as such.  The entropy consumption is really a 
> libgcrypt issue; it does not make much sense to work around it in each 
> application individually.

Let me sum up all the pieces to see, if I get everything in the right way.

a) Exim uses GnuTLS in a way, which causes it to use /dev/random to aquire 
   strictly random bytes on every encrypted connection
b) GnuTLS uses much more random bytes to initialize itself than OpenSSL.

A combination of A and B leads to a possible hole for a DoS attack, if I 
am able to drain the entropy pool, because of A, Exim will block until 
enough entropy is regathered.

While fixing A is generally a Good Idea (in my opinion), so Exim does not 
block if there is no entropy available (because of whatever reason).

But the real winner would be a fix to B so that the entropy pool does not 
get drained so fast and would not only benefit Exim but any other program 
using GnuTLS.

Is this summary correct so far?

Grüße,
Sven.

-- 
Sven Hartge -- professioneller Unix-Geek
Meine Gedanken im Netz: http://www.svenhartge.de/

Achtung, neue Mail-Adresse: sven at svenhartge.de