Re: [PATCH v5] ELF: implement AT_RANDOM for glibc PRNG seeding

From: Ulrich Drepper
Date: Tue Oct 21 2008 - 16:23:20 EST

Hash: SHA1

Andrew Morton wrote:
> I read the above changeloglet and read the above-linked page and it's
> still 87% unclear to me what this feature does. Something to do with
> stack randomisation, apparently. I suppose I could go do further
> hunting, but from the quality-of-changelog POV I don't think I should
> need to do so.

Not stack randomization. glibc needs right after startup a bit of
random data for internal protections (stack canary etc). What is now in
upstream glibc is that we always unconditionally open /dev/urandom, read
some data, and use it. For every process startup. That's slow.

In addition Andi mentioned that this use of /dev/urandom might be
problematic. I let him explain this.

The solution is to provide a limited amount of random data to the
starting process in the aux vector. I suggested 16 bytes and this is
what the patch implements. If we need only 16 bytes or less we use the
data directly. If we need more we'll use the 16 bytes to see a PRNG.
This avoids the costly /dev/urandom use and it allows the kernel to use
the most adequate source of random data for this purpose. It might not
be the same pool as that for /dev/urandom.

> It's unclear to me that the random-number issue got sorted out?

I think the last patch used the normal function to get 16 random bytes,
equivalent to the data used for stack randomization etc.

If Andi has concrete proposals for a revamp of the use of entropy in the
kernel this can be easily done as an add-on. This patch doesn't make
the situation worse, it doesn't deplete entropy more than it happens now.

- --
â Ulrich Drepper â Red Hat, Inc. â 444 Castro St â Mountain View, CA â
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