linux-kernel - Re: repeatable boot randomness inside KVM guest

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Date:   Tue, 17 Apr 2018 10:13:34 +0100
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Matthew Wilcox <willy@...radead.org>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Alexey Dobriyan <adobriyan@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: repeatable boot randomness inside KVM guest

On Sat, 2018-04-14 at 17:41 -0700, Matthew Wilcox wrote:
> On Sat, Apr 14, 2018 at 06:44:19PM -0400, Theodore Y. Ts'o wrote:
> > What needs to happen is freelist should get randomized much later
> > in the boot sequence.  Doing it later will require locking; I don't
> > know enough about the slab/slub code to know whether the slab_mutex
> > would be sufficient, or some other lock might need to be added.
> 
> Could we have the bootloader pass in some initial randomness?

Where would the bootloader get it from (securely) that the kernel
can't?  For example, if you compile in a TPM driver, the kernel will
pick up 32 random entropy bytes from the TPM to seed the pool, but I
think it happens too late to help with this problem currently.  IMA
also needs the TPM very early in the boot sequence, so I was wondering
about using the initial EFI driver, which is present on boot, and then
transitioning to the proper kernel TPM driver later, which would mean
we could seed the pool earlier.

As long as you mix it properly and limit the amount, it shouldn't
necessarily be a source of actual compromise, but having an external
input to our cryptographically secure entropy pool is an additional
potential attack vector.

James