Re: [RFC PATCH] x86/entry/64: randomize kernel stack offset upon syscall

[Andy Lutomirski]

> On Mar 28, 2019, at 8:45 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 
>> On Tue, Mar 26, 2019 at 9:31 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> 
>> On Tue, Mar 26, 2019 at 3:35 AM Reshetova, Elena
>> <elena.reshetova@xxxxxxxxx> wrote:
>>> 
>>>>> On Mon, Mar 18, 2019 at 1:16 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>>> On Mon, Mar 18, 2019 at 2:41 AM Elena Reshetova
>>>>> <elena.reshetova@xxxxxxxxx> wrote:
>>>>>> Performance:
>>>>>> 
>>>>>> 1) lmbench: ./lat_syscall -N 1000000 null
>>>>>>    base:                     Simple syscall: 0.1774 microseconds
>>>>>>    random_offset (rdtsc):     Simple syscall: 0.1803 microseconds
>>>>>>    random_offset (rdrand): Simple syscall: 0.3702 microseconds
>>>>>> 
>>>>>> 2)  Andy's tests, misc-tests: ./timing_test_64 10M sys_enosys
>>>>>>    base:                     10000000 loops in 1.62224s = 162.22 nsec / loop
>>>>>>    random_offset (rdtsc):     10000000 loops in 1.64660s = 164.66 nsec / loop
>>>>>>    random_offset (rdrand): 10000000 loops in 3.51315s = 351.32 nsec / loop
>>>>>> 
>>>>> 
>>>>> Egads!  RDTSC is nice and fast but probably fairly easy to defeat.
>>>>> RDRAND is awful.  I had hoped for better.
>>>> 
>>>> RDRAND can also fail.
>>>> 
>>>>> So perhaps we need a little percpu buffer that collects 64 bits of
>>>>> randomness at a time, shifts out the needed bits, and refills the
>>>>> buffer when we run out.
>>>> 
>>>> I'd like to avoid saving the _exact_ details of where the next offset
>>>> will be, but if nothing else works, this should be okay. We can use 8
>>>> bits at a time and call prandom_u32() every 4th call. Something like
>>>> prandom_bytes(), but where it doesn't throw away the unused bytes.
>>> 
>>> Actually I think this would make the end result even worse security-wise
>>> than simply using rdtsc() on every syscall. Saving the randomness in percpu
>>> buffer, which is probably easily accessible and can be probed if needed,
>>> would supply attacker with much more knowledge about the next 3-4
>>> random offsets that what he would get if we use "weak" rdtsc. Given
>>> that for a successful exploit, an attacker would need to have stack aligned
>>> once only, having a knowledge of 3-4 next offsets sounds like a present to an
>>> exploit writer...  Additionally it creates complexity around the code that I
>>> have issues justifying with "security" argument because of above...
> 
> That certainly solidifies my concern against saving randomness. :)
> 
>>> I have the patch now with alloca() and rdtsc() working, I can post it
>>> (albeit it is very simple), but I am really hesitating on adding the percpu
>>> buffer randomness storage to it...
>>> 
>> 
>> Hmm.  I guess it depends on what types of attack you care about.  I
>> bet that, if you do a bunch of iterations of mfence;rdtsc;syscall,
>> you'll discover that the offset between the user rdtsc and the
>> syscall's rdtsc has several values that occur with high probability.
> 
> How about rdtsc xor with the middle word of the stack canary? (to
> avoid the 0-byte) Something like:
> 
>    rdtsc
>    xorl [%gs:...canary....], %rax
>    andq  $__MAX_STACK_RANDOM_OFFSET, %rax
> 
> I need to look at the right way to reference the canary during that
> code. Andy might know off the top of his head. :)
> 

Doesnât this just leak some of the canary to user code through side channels?