Re: [PATCH 2/2] ima: Support appraise_type=imasig_optional

From: Mimi Zohar
Date: Thu Feb 14 2013 - 12:02:51 EST

On Thu, 2013-02-14 at 09:40 -0500, Vivek Goyal wrote:
> On Wed, Feb 13, 2013 at 04:45:23PM -0500, Mimi Zohar wrote:

> > > If it would happen that it contains signature, then IMA_DIGSIG flag
> > > would be set,
> > > and process could get needed capability as Vivek wants.
> >
> > With the 'optional' condition, both unsigned and validly signed files
> > will succeed. One way of making this information accessible to an LSM,
> > would be to define a new integrity capability and set it here. The new
> > integrity capability would indicate the file was validly signed.
> Thinking loud.
> The problem with integrity capability is that it goes only so far. If
> we provide capability in exec() path, then that capability means much
> more in the sense, we know file is locked to run from memory. An integrity
> capability just means file is validly signed.

> So exec() code might have to do another capability on top which will
> also ensure that file is executable is locked in memory and signature
> verification is done after loading in memory so that it is not open
> to writing to disk block attacks.
> And based on this capability we probably need to deny write access to file
> till file is open for exec() (I noticed that after load, we seem to be
> allowing access to write access).

I think we're back to my original comment that the bprm_check hook might
need to be moved or an additional hook added, as the existing bprm_check
hook is located before the file is locked from modification. :)



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