Re: [PATCH v2] x86: Disable kernel stack offset randomization for !TSC

[Ingo Molnar <mingo@kernel.org>]

* Maciej W. Rozycki <macro@orcam.me.uk> wrote:

> For x86 kernel stack offset randomization uses the RDTSC instruction, 
> which causes an invalid opcode exception with hardware that does not 
> implement this instruction:

> @@ -85,7 +86,8 @@ static inline void arch_exit_to_user_mod
>  	 * Therefore, final stack offset entropy will be 5 (x86_64) or
>  	 * 6 (ia32) bits.
>  	 */
> -	choose_random_kstack_offset(rdtsc() & 0xFF);
> +	if (cpu_feature_enabled(X86_FEATURE_TSC))
> +		choose_random_kstack_offset(rdtsc() & 0xFF);
>  }

While this is an obscure corner case, falling back to 0 offset silently 
feels a bit wrong - could we at least attempt to generate some 
unpredictability in this case?

It's not genuine entropy, but we could pass in a value that varies from 
task to task and which is not an 'obviously known' constant value like the 
0 fallback?

For example the lowest 8 bits of the virtual page number of the current 
task plus the lowest 8 bits of jiffies should vary from task to task, has 
some time dependence and is cheap to compute:

	(((unsigned long)current >> 12) + jiffies) & 0xFF

This combined with the per-CPU forward storage of previous offsets:

#define choose_random_kstack_offset(rand) do {                          \
        if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
                                &randomize_kstack_offset)) {            \
                u32 offset = raw_cpu_read(kstack_offset);               \
                offset ^= (rand);                                       \
                raw_cpu_write(kstack_offset, offset);                   \
        }                                                               \

Should make this reasonably hard to guess for long-running tasks even if 
there's no TSC - and make it hard to guess even for tasks whose creation an 
attacker controls, unless there's an info-leak to rely on.

Thanks,

	Ingo