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    SubjectRe: [PATCH v5 00/21] EVM
    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

    > > 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.



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