Bug#387448: empty entropy pool leads to DOS

[Andreas Metzler]

On 2006-09-14 Yuri D'Elia <wavexx at yuv.info> wrote:
> Package: exim4
> Version: 4.63-3
> Severity: important

> I know this has been reported before to death [since gnutls is being used],
> but I will just add another twist, since I'm tired of rebuilding exim with
> OpenSSL manually.

> GnuTLS drains the entropy pool much more quickly than OpenSSL. On server
> systems without hardware generators, /dev/random drains very quickly, meaning
> that exim will often block. But exim should NOT block, or even wait, in
> STARTTLS. It is possible to make the system drain its entropy and then issue
> several connections all waiting in STARTTLS, until the maximal number of
> connection is reached. Combine this with the fact that it is possible to
> maintain the connection alive for eternity with a SO_KEEPALIVE connection, and
> also exim doesn't seem to terminate the process when the connection is closed
> in this state, and you get very easy denial of service which will refuse all
> further (including normal) connections.

> This is a bug in exim. exim should NOT block in STARTTLS. keys must be
> generated in background or by other means, and the unavailability of data at
> STARTTLS should generate and immediate temporary failure to avoid other DOS
> conditions.

Hello,
Do you have gnutls-bin installed at all?

The only thing causing exim to block on STARTTLS is key and dh-param
generation. Both is done offline (/etc/cron.daily/exim4-base invoking
/usr/share/exim4/exim4_refresh_gnutls-params which uses certtool).

Normal STARTTLS connection does not block, as it only reads from
/dev/urandom.

--------------
ametzler at argenau:/tmp$ cat /proc/sys/kernel/random/entropy_avail ; for i in 1 2 3 4 ; do swaks -tls --silent -s localhost -q ehlo ; cat /proc/sys/kernel/random/entropy_avail ; done
3953
1
1
1
1
--------------
--------------
argenau:/tmp/GNUTLS11# strace -v -f -s 164 -eopen -p 2455                  
Process 2455 attached - interrupt to quit
Process 12999 attached
[pid 12999] open("/etc/hosts", O_RDONLY) = 3
[pid 12999] open("/etc/hosts", O_RDONLY) = 3
[pid 12999] open("/var/spool/exim4/gnutls-params", O_RDONLY|O_LARGEFILE) = 3
[pid 12999] open("/etc/exim4/exim.key", O_RDONLY) = 3
[pid 12999] open("/etc/exim4/exim.crt", O_RDONLY) = 3
[pid 12999] open("/dev/urandom", O_RDONLY) = 3
Process 12999 detached
--- SIGCHLD (Child exited) @ 0 (0) ---
Process 13003 attached
[pid 13003] open("/etc/hosts", O_RDONLY) = 3
[pid 13003] open("/etc/hosts", O_RDONLY) = 3
[pid 13003] open("/var/spool/exim4/gnutls-params", O_RDONLY|O_LARGEFILE) = 3
[pid 13003] open("/etc/exim4/exim.key", O_RDONLY) = 3
[pid 13003] open("/etc/exim4/exim.crt", O_RDONLY) = 3
[pid 13003] open("/dev/urandom", O_RDONLY) = 3
Process 13003 detached
--- SIGCHLD (Child exited) @ 0 (0) ---
Process 13007 attached
[pid 13007] open("/etc/hosts", O_RDONLY) = 3
[pid 13007] open("/etc/hosts", O_RDONLY) = 3
[pid 13007] open("/var/spool/exim4/gnutls-params", O_RDONLY|O_LARGEFILE) = 3
[pid 13007] open("/etc/exim4/exim.key", O_RDONLY) = 3
[pid 13007] open("/etc/exim4/exim.crt", O_RDONLY) = 3
[pid 13007] open("/dev/urandom", O_RDONLY) = 3
Process 13007 detached
--- SIGCHLD (Child exited) @ 0 (0) ---
Process 13011 attached
[pid 13011] open("/etc/hosts", O_RDONLY) = 3
[pid 13011] open("/etc/hosts", O_RDONLY) = 3
[pid 13011] open("/var/spool/exim4/gnutls-params", O_RDONLY|O_LARGEFILE) = 3
[pid 13011] open("/etc/exim4/exim.key", O_RDONLY) = 3
[pid 13011] open("/etc/exim4/exim.crt", O_RDONLY) = 3
[pid 13011] open("/dev/urandom", O_RDONLY) = 3
Process 13011 detached
--- SIGCHLD (Child exited) @ 0 (0) ---
--------------
cu andreas
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