Re: [PATCH 2/2] x86: Allow disabling of sys_iopl, sys_ioperm

Mike Waychison
2011-07-15T00:49:15Z

On Thu, Jul 14, 2011 at 4:39 PM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
>> In my case, I simply don't want these "features", which is why I took
>> the compile time approach to turning this stuff off.  I realize that
>> these syscalls (and the /dev/port interface) are not comprehensive (I
>> didn't say they were either).  I'm happy though to take suggestions
>
> Indeed - but from the point of view of doing the job to a standard for
> an upstream kernel there ought to be a meaningful testable definition of
> what the security shift you achieve is.
>
>> for stuff I probably should be disabling considering my goal of making
>> it difficult for root to compromise a system.  And yes, modules are
>> disabled :)
>
> If you have CAP_SYS_RAWIO and some of the other interfaces you only think
> it is - the kiddies toolkits already include out of the box direct module
> loading hacks (in fact its fairly easy if you've got GPU PCI access to
> just put the module into video memory so that only the patching needs to
> be done and the module internal addresses are all fixed and can be
> arranged on a suitably convenient target address)
>
> So really there needs to be a definition of what you are trying to
> achieve. My own guess is that for
>
>        "Disallow direct access paths to hardware"
>
> the actual answer is 'revoke RAWIO' and then give it back to very
> specific selected code in very specific selected ways or possibly in some
> cases where rawio is needed for stuff that shouldn't be by writing new
> driver bits to provide the proper interface that we are lacking ?
>
> So lets turn the question around a moment - what breaks if you simply
> take CAP_SYS_RAWIO away from everything ?
>

Alright, I see your point. ISTR that CAP_SYS_RAWIO was required for
accessing block devices directly, but this doesn't seem to be the
case.

I think the approach I'll try next is to try and drop it with
PR_CAPBSET_DROP from early userspace's init.

Any other vectors you would suggest to keep the kiddies away?