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

[Mimi Zohar]

On Thu, 2013-02-14 at 15:57 -0500, Vivek Goyal wrote:
> On Thu, Feb 14, 2013 at 03:54:45PM -0500, Vivek Goyal wrote:
> > On Thu, Feb 14, 2013 at 02:49:16PM -0500, Mimi Zohar wrote:
> > 
> > [..]
> > > > > I think you're making this more complicated than it needs to be.  Allow
> > > > > the execution unless the file failed signature verification.  The
> > > > > additional capability is given only if the signature verification
> > > > > succeeds.
> > > > 
> > > > I am just trying to bring it inline with module signature verification.
> > > > There also module loading fails if signatures are present but kernel
> > > > can't verify it.
> > > 
> > > A specific hook is defined for kernel module signature verification,
> > > which is enabled/disabled in Kconfig.  When enabled, only signed modules
> > > are loaded.  The kernel module hook does not verify the integrity of the
> > > userspace application (eg. insmod, modprobe), but of the kernel module
> > > being loaded.
> > > 
> > > Your original patches verified the integrity of the userspace
> > > application kexec, not the image being loaded.  ima_bprm_check()
> > > verifies the integrity of executables.  To permit both signed and
> > > unsigned files to execute, we defined the 'optional' IMA policy flag,
> > > with the intention of giving more capability to signed executables.
> > > 
> > > Unless we define a kexec specific hook for verifying kernel images, it's
> > > not the same.
> > 
> > I think we are talking of two different things here.
> > 
> > I am referring to kernel module signing where signatures are appended
> > to module (not IMA hook).
> > 
> > Also I am just referring to behavior about what happens if some error
> > happens while signature verification.
> > 
> > - If signature verification fails, it is clear what to do.
> > - If signature verification passes, it is clear what to do.
> > - Grey area is, what happens if some error is encountered during signature
> >   verification. Should the module loading be allowed/disallowed. Looking
> >   at the module loading code, once it is determined that module has
> >   signature appended to it, module loading fails if some error occurs
> >   during signature verification.

There better not be any gray area, if CONFIG_MODULE_SIG_FORCE is
enabled, only validly signed modules should be loaded.

> > So I am just referring to that fact and trying to draw parallels between
> > error handling during module signature verification and error handling
> > when file appraisal happens in IMA. 

> > There can be two options.
> > 
> > - Disallow execution only if signature verification fails. If some error
> >   happens during verification, ignore it, let the executable continue.
> >   Just that it does not get extra capability.
> > 
> > - Disallow execution only if executable is not signed or it has valid
> >   signature. If executable is signed and some error happens during the
> >   process of verifying signature, execution is denied.
> > 
> 
> Little typo in second option. I meant "Allow execution only if executable
> is not signed or it has valid signatures".

Executables will be run with or without a valid signature.  The only
fair comparison would be between loading the kernel module and setting a
capability.  Both are only done based on a valid signature.  Of the two
options, I would choose the second.

Mimi


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