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    SubjectRe: CONFIG_RANDOM (compromise?)
       Date: Mon, 20 May 1996 12:00:27 +0200
    From: Harald Anlauf <>

    [This mail is not CC'ed to the linux-kernel list]

    Actually, yes it was....

    If somebody (else) runs a process constantly sucking numbers from
    /dev/random on an (maybe your) essentially "idle" machine, i.e. with
    little activity on keyboard, disk, network, etc., can you still
    guarantee that _you_ still get sufficiently good random numbers from
    /dev/random, to prevent any attacks, even if this "somebody else"
    communicates these numbers to an assumed attacker? (Do not assume that
    you can use e.g. the Pentium time stamp register).

    If you have a "bad guy" running on your machine, they can constantly
    suck numbers from /dev/random. This will cause a "denial of service
    attack", since /dev/random will only issue random numbers if sufficient
    entropy is available to generate them.

    So an application which uses /dev/random can block; if the application
    does not want to block, it can open /dev/random in non-blocking mode
    (usually recommended). However, this does not answer the question of
    what to do when /dev/random has been exhausted. The right course of
    action is probably to give the user a warning message and exit.

    While this may not sounds entirely satisfactory, consider what else an
    attacker to could if they have access to your machine. (a) they could
    try breaking in as root, (b) they could do resource starvation by
    running a program which does:

    while (1) {
    cp = malloc(1 megabyte);

    (c) they could break in using some neglected hole in (pick your choice
    of) sendmail, /proc, NIS, NFS, etc., etc., etc.

    In the long run, a system which is doing fair-resource allocation to
    prevent one user from grabbing all availble CPU, virtual memory, and
    other resources will also have to treat /dev/random as a valuable
    resource whose use must be controlled to prevent one user from grabbing
    all available entropy. However, this sort of resource control is hard
    to do right; especially if you want an efficient system! Given that we
    don't even handle memory exhaustion terribly efficiently at the moment,
    random number exhaustion is a similar (unsolved) problem in Linux.

    When we solve the general resource allocation problem, it should not be
    terribly difficult to extend it to solve the /dev/random allocation
    problem. Why hasn't it been addressed in Linux so far? I suspect
    because there aren't that many Linux systems doing serious time-sharing
    systems. We have machines which are network services, and single-user
    desktop machines, but for those machines things like quotas and resource
    allocation aren't as important. While there are some time-sharing
    machiens using Linux, they tend to be in the minority.

    - Ted

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