Re: [PATCH] kernel: make /proc/kallsyms mode 400 to reduce ease of attacking

["H. Peter Anvin" <hpa@zytor.com>]

On 11/10/2010 12:53 AM, Ingo Molnar wrote:
> 
> * H. Peter Anvin <hpa@zytor.com> wrote:
> 
>> We already do virtual relocation on 32 bits, and replicating that on 64 bits 
>> wouldn't be hard.  However, the linkage script strongly assumes congruency mod 2/4 
>> MiB, and that is probably nontrivial to change.  However, that still gives about 9 
>> bits of entrophy to play with.  The question is if that is enough, or if we'd have 
>> to do more clever hacks.
> 
> Even 1 bit of entropy would bring a visible improvement: a failed exploit attempt to 
> the wrong address can crash the kernel with a 50% chance. 9 bits would be very nice.
> 
> If an exploit can be brute-forced without crashing the kernel then only some 
> significantly large bitness would help. So while 9 bits would be rather low for a 
> user-space ASLR scheme [many user-space bugs can be brute-forced without crashing 
> the system and raising alarms], it's very attractive for kernel ASLR.
> 

Now, *relative* symbol addresses will typically not have any randomness
at all, which may limit the usefulness, of course.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.