Re: [PATCH v5 00/21] EVM

[Pavel Machek <pavel@ucw.cz>]

On Fri 2011-05-27 13:45:51, David Safford wrote:
> On Thu, 2011-05-26 at 22:17 +0200, Pavel Machek wrote:
> 
> > I suggest you explain the patchset in the emails, then? Everyone here
> > seems to be confused... Attack it protects against, and what kind of
> > hardware is needed for the protection to be effective?
> 
> The white paper is over 15 pages, and it barely scratches the surface.
> Every customer has different security threat models and requirements.
> Discussing this in general on the mailing list is really hard.
> 
> So let's try to simplify this just down to digital signatures in
> the cellphone environment, as you state:

Good.

> > Because AFAICT, file signatures, as proposed, are only useful for
> > locking down my cellphone against myself. (That's -- evil).
> 
> The proposed digital signatures can enforce authenticity of a file's 
> data (IMA-Appraisal with Digital Signature), and of a file's metadata
> (EVM with Digital Signature). For most users, enforcing authenticity
> of files is a good thing - a user knows that they are running authentic
> software signed by their phone manufacturer, and not malicious files
> that they, or someone else installed. In this threat model, EVM is 

Ok, so lets talk about smartphone, similar to my HTC Dream (developer
version, unlocked bootloader, flashable from kernel (*)).

Yes, I could install the crazy EVM/IMA infastructure to prevent
applications modifying selected files.

But... I could just do chattr +i on selected files, I do not need
fancy EVM/IMA for that.

> Blocking signature verification would serve only to punish Linux 
> users who care about the authenticity of their files, while doing 
> _nothing_ to stop manufacturers from locking their bootloaders.

chattr already protects authenticity of my files, as do standard unix
permissions.

So... where's the difference?

								Pavel
(*) but it does not change anything.

True; determined attacker could steal my cellphone, open it up,
desolder the flash, and change attributes of the filesystem.

But... the same determined attacker can also replace
bootloader&kernel&filesystem -- that is in the same flash! -- with
unlocked versions. So the argumentation is the same for locked down
phone.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html