Re: [RFC][PATCH] Randomize kernel base address on boot

Dan Rosenberg
2011-05-27T13:21:46Z

On Fri, 2011-05-27 at 09:13 -0400, Vivek Goyal wrote:
> On Thu, May 26, 2011 at 04:44:34PM -0400, Dan Rosenberg wrote:
> > On Thu, 2011-05-26 at 16:40 -0400, Vivek Goyal wrote:
> > > On Thu, May 26, 2011 at 04:35:02PM -0400, Vivek Goyal wrote:
> > > > On Tue, May 24, 2011 at 04:31:45PM -0400, Dan Rosenberg wrote:
> > > > > This introduces CONFIG_RANDOMIZE_BASE, which randomizes the address at
> > > > > which the kernel is decompressed at boot as a security feature that
> > > > > deters exploit attempts relying on knowledge of the location of kernel
> > > > > internals. The default values of the kptr_restrict and dmesg_restrict
> > > > > sysctls are set to (1) when this is enabled, since hiding kernel
> > > > > pointers is necessary to preserve the secrecy of the randomized base
> > > > > address.
> > > >
> > > > What happens to /proc/iomem interface which gives us the physical memory
> > > > location where kernel is loaded. kexec-tools relies on that interface
> > > > heavily so we can not take it away. And if we can not take it away then
> > > > I think somebody should be easibly be able to calculate this randomized
> > > > base address.
> >
> > Is it common to run kexec-tools as non-root? It may be necessary to
> > restrict this interface to root when randomization is used (keep in mind
> > nobody's going to force you to turn this on by default, at least for the
> > foreseeable future).
>
> Dan,
>
> I had a stupid question. /proc/kallsyms is also readable by root only. So
> if we are doing this so that non-root user can not know kernel virtual and
> physical address that should be already covered as non-root users can't
> read /proc/kallsysm or /boot/System.map.
>

Not sure what system you're running, but /proc/kallsyms is 0444 on my
machine (and in mainline, afaik). Likewise for /proc/iomem.

The problem is mainly with distribution kernels - it's trivial to just
grab an identical vmlinux to a target machine and then you instantly
know exactly where everything is.

> And if this randomization is also to protect information from root user
> then /proc/iomem exporting the physical address of kernel is still a
> valid question in that context.
>

I think we can deal with unprivileged users first, and if we want to
truly prevent root from finding this out, we can introduce a separate
toggle that locks things down further.

-Dan

> Thanks
> Vivek