Re: [PATCH v5 00/21] EVM

Mimi Zohar
2011-05-31T12:05:18Z

On Sun, 2011-05-29 at 08:58 +0200, Pavel Machek wrote:
> 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.

For files that you don't expect to change, such as ELF executables, you
probably could use the immutable flag, but using a digital signature
provides authenticity as well, which the immutable flag does not
provide.

> > 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

Neither digital signatures nor the immutable flag work for files that
change, such as config files. For these files, ima-appraisal would
store a file hash.

> (*) 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.

With EVM, assuming that i_flag is included in the EVM HMAC, which it
currently isn't, you would be able to detect the change and prevent the
file from being accessed.

>
> 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.
>

As EVM is not involved in the boot process, it can not and does not
address this, but other technologies could.

thanks,

Mimi