linux-kernel - Re: [BUG] perf: bogus correlation of kernel symbols

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Date:	Fri, 20 May 2011 15:11:08 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Dan Rosenberg <drosenberg@...curity.com>
Cc:	linux-kernel@...r.kernel.org, davej@...hat.com,
	kees.cook@...onical.com, davem@...emloft.net, eranian@...gle.com,
	torvalds@...ux-foundation.org, adobriyan@...il.com,
	penberg@...nel.org
Subject: Re: [BUG] perf: bogus correlation of kernel symbols


* Dan Rosenberg <drosenberg@...curity.com> wrote:

> On Fri, 2011-05-20 at 14:07 +0200, Ingo Molnar wrote:
> > * Dan Rosenberg <drosenberg@...curity.com> wrote:
> > 
> > > I was able to boot a relocatable kernel with the decompression location at a 
> > > hard-coded offset without too much trouble.  Everything seems to work fine.
> > 
> > Nice!
> > 
> > > However, it occurred to me that even if the kernel image's base address were 
> > > randomized at boot, assuming a binary distro kernel it would still be 
> > > possible to sidt the address of the IDT and calculate symbol offsets relative 
> > > to that.  Any thoughts on how to avoid that?  Seems difficult. Another hurdle 
> > > will be to find a reasonable source of entropy that early in the boot 
> > > process.
> > 
> > I do not think it's an issue.
> > 
> > If an attacker can execute arbitrary privileged instructions like SIDT then 
> > it's game over. There's plenty of CPU state, the IDT, GDT, various MSRs 
> > that would tell roughly where the kernel is, etc.
> 
> Except that SIDT isn't a privilege instruction, it's accessible as ring 3.

Oops, stupid me :-/

We need to allocate the IDT dynamically: just kmalloc() it, update idt_descr 
and do a load_idt(). Double check places that modify idt_descr or use 
idt_table.

Note, you could do this as a side effect of a nice performance optimization: 
would you be interested in allocating it in the percpu area, using 
percpu_alloc()? That way the IDT is distributed between CPUs - this has 
scalability advantages on NUMA systems and maybe even on SMP.

> > The attack randomization protects against is when the attacker has a 
> > limited amount of control over a stack return address (due to a buffer 
> > overflow for example) and can redirect kernel execution to some 
> > 'interesting' place that allows more control. With SMEP and kernel image 
> > randomization this would be rather difficult to pull off: the kernel wont 
> > jump to a pre-prepared user-space shellcode buffer (due to SMEP) while the 
> > location of already existing, executable, supervisor-privileged pages is 
> > randomized.
> 
> Yes, all true, except are you specifically considering remote-only attack 
> vectors?

No, unprivileged local user, so yes, the IDT address has to be protected.

Thanks,

	Ingo
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