On Fri, 2 Jan 2026 13:11:54 +0000
Ryan Roberts <[email protected]> wrote:
> Previously different architectures were using random sources of
> differing strength and cost to decide the random kstack offset. A number
> of architectures (loongarch, powerpc, s390, x86) were using their
> timestamp counter, at whatever the frequency happened to be. Other
> arches (arm64, riscv) were using entropy from the crng via
> get_random_u16().
>
> There have been concerns that in some cases the timestamp counters may
> be too weak, because they can be easily guessed or influenced by user
> space. And get_random_u16() has been shown to be too costly for the
> level of protection kstack offset randomization provides.
>
> So let's use a common, architecture-agnostic source of entropy; a
> per-cpu prng, seeded at boot-time from the crng. This has a few
> benefits:
>
> - We can remove choose_random_kstack_offset(); That was only there to
> try to make the timestamp counter value a bit harder to influence
> from user space.
>
> - The architecture code is simplified. All it has to do now is call
> add_random_kstack_offset() in the syscall path.
>
> - The strength of the randomness can be reasoned about independently
> of the architecture.
>
> - Arches previously using get_random_u16() now have much faster
> syscall paths, see below results.
>
> There have been some claims that a prng may be less strong than the
> timestamp counter if not regularly reseeded. But the prng has a period
> of about 2^113. So as long as the prng state remains secret, it should
> not be possible to guess. If the prng state can be accessed, we have
> bigger problems.
If you have 128 bits of output from consecutive outputs I think you
can trivially determine the full state using (almost) 'school boy' maths
that could be done on pencil and paper.
(Most of the work only has to be done once.)
The underlying problem is that the TAUSWORTHE() transformation is 'linear'
So that TAUSWORTHE(x ^ y) == TAUSWORTHE(x) ^ TAUSWORTHE(y).
(This is true of a LFSR/CRC and TOUSWORTH() is doing some subset of CRCs.)
This means that each output bit is the 'xor' of some of the input bits.
The four new 'state' values are just xor of the the bits of the old ones.
The final xor of the four states gives a 32bit value with each bit just
an xor of some of the 128 state bits.
Get four consecutive 32 bit values and you can solve the 128 simultaneous
equations (by trivial substitution) and get the initial state.
The solution gives you the 128 128bit constants for:
u128 state = 0;
u128 val = 'value returned from 4 calls';
for (int i = 0; i < 128; i++)
state |= parity(const128[i] ^ val) << i;
You done need all 32bits, just accumulate 128 bits.
So if you can get the 5bit stack offset from 26 system calls you know the
value that will be used for all the subsequent calls.
Simply changing the final line to use + not ^ makes the output non-linear
and solving the equations a lot harder.
I might sit down tomorrow and see if I can actually code it...
David
