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

[Kees Cook]

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. :)

-Kees

-- 
Kees Cook