Re: Reading large amounts from /dev/urandom broken

From: Alex Elsayed
Date: Thu Jul 24 2014 - 16:39:51 EST

Hannes Frederic Sowa wrote:

> On Mi, 2014-07-23 at 11:14 -0400, Theodore Ts'o wrote:
>> On Wed, Jul 23, 2014 at 04:52:21PM +0300, Andrey Utkin wrote:
>> > Dear developers, please check bugzilla ticket
>> > (not the initial
>> > issue, but starting with comment#3.
>> >
>> > Reading from /dev/urandom gives EOF after 33554431 bytes. I believe
>> > it is introduced by commit 79a8468747c5f95ed3d5ce8376a3e82e0c5857fc,
>> > with the chunk
>> >
>> > nbytes = min_t(size_t, nbytes, INT_MAX >> (ENTROPY_SHIFT + 3));
>> >
>> > which is described in commit message as "additional paranoia check to
>> > prevent overly large count values to be passed into urandom_read()".
>> >
>> > I don't know why people pull such large amounts of data from urandom,
>> > but given today there are two bugreports regarding problems doing
>> > that, i consider that this is practiced.
>> I've inquired on the bugzilla why the reporter is abusing urandom in
>> this way. The other commenter on the bug replicated the problem, but
>> that's not a "second bug report" in my book.
>> At the very least, this will probably cause me to insert a warning
>> printk: "insane user of /dev/urandom: [current->comm] requested %d
>> bytes" whenever someone tries to request more than 4k.
> Ok, I would be fine with that.
> The dd if=/dev/urandom of=random_file.dat seems reasonable to me to try
> to not break it. But, of course, there are other possibilities.

Personally, I'd say that _is_ insane - reading from urandom still consumes
entropy (causing readers of /dev/random to block more often); when
alternatives (such as dd'ing to dm-crypt) both avoid the issue _and_ are
faster then it should very well be considered pathological.

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