linux-kernel - Re: [PATCH RESEND 1/1] crypto API: RSA algorithm patch (kernel version 2.6.20.1)

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Message-ID: <460141B4.1080109@sciensis.com>
Date:	Wed, 21 Mar 2007 16:31:16 +0200
From:	Tasos Parisinos <t.parisinos@...ensis.com>
To:	Indan Zupancic <indan@....nu>
Cc:	Francois Romieu <romieu@...zoreil.com>,
	herbert@...dor.apana.org.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 1/1] crypto API: RSA algorithm patch (kernel  
              version 2.6.20.1)

Indan Zupancic wrote:
> On Wed, March 21, 2007 14:07, Tasos Parisinos wrote:
>   
>>> On Wed, March 21, 2007 10:15, Tasos Parisinos wrote:
>>>
>>>       
>>>> Protecting a TripleDES key in high security standards is not as
>>>> simple as making the kernel read
>>>> protected, you need a whole lot and
>>>> that also means hardware (cryptomemories e.t.c)
>>>> So you forget about all this overhead when you use assymetric
>>>>
>>>>         
>>> Ah, you're talking about fishing the key out of RAM here, right?
>>> My point stays the same for that: If you can't read protect the
>>> kernel RAM, small chance you can write protect it. And then they
>>> can just bypass all signature checking you put in it anyway.
>>>
>>>       
>> How can one tamper (write) the kernel memory of a booted and running kernel
>> without using an exploitable bug?
>>
>> I mean, you can't mess with the bzImage on flash, the secure bootloader
>> boots it without
>> letting someone alter the (non crypto-) memory while loading the bzImage
>> on it, and then
>> no-one can run something that will tamper the system or write anywhere
>> on kernel memory
>> without exploiting a bug
>>
>> I mean, am i missing something here?
>>     
>
> Depends on what you consider an exploitable bug. Does getting root access count?
> If not, then you must make very sure that all possible ways to modify kernel
> memory from userspace are thwarted (I don't know what those are, hopefully
> loading modules is the only one, but maybe there are smart other ways).
>
> Assuming one can't write kernel memory, it's also safe to assume kernel memory
> can be protected against reads (else the whole keys infrastructure is useless).
>
> But instead of only reading the bus traffic also modifying it doesn't seem so
> far fetched to me. That modification can be reduced to swapping one bit which
> tells whether the modules being loaded has a valid signature or not.
> Timing it might be tricky, but that can be automated.
>
> When someone has the hardware in his hands and really want to exploit it, you
> lose no matter what you do. At best you can make it harder and more expensive.
>
> In the end my point is that you might think that you can get away with less
> security when using RSA, but perhaps in reality you don't. At least when using
> symmetric key encryption you're forced to secure the whole thing more.
>
> So design it for symmetric keys. If it turns out that using asymmetric keys is
> more practical for whatever reason, fine, use those. But they won't give you
> added security.
>
> Greetings,
>
> Indan
>
>
> P.S. The whole argument of secure bootloader checks the kernel can be
> extrapolated to a secure kernel checking a user space program. Why not
> letting the kernel check the signature of a monolithic modprobe program,
> and let it do all the (RSA) checking. The expected hash of the modprobe
> program can be hardcoded in the kernel.
>
>
>
>   
I agree that you have no more security that using symmetric
but we believe you have lower costs, simpler key management
(which is a big headache alone), tougher to break through
(not unbreakable) and more centralization

As for modprobe u are right but we also need to check (apart
from kernel modules) the executables and libraries in the
usage scenario.

About time:
In my pc system running 2.6.20.3 (2.66 GHz P4, 1G mem) the computation of
modular exponentiation of 1Kbit (with a 32bit exponent all bits set and 
a 1024 bit key)
took almost 3ms. That's the time needed to check the signature of any 
code loaded
in ram using this module, after having it hashed (sha1) and signature 
extracted from elf.

Best regards
Tasos Parisinos
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