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

[Ingo Molnar]

* Maciej W. Rozycki <macro@xxxxxxxxxxx> wrote:

> Jason,
> 
>  Would you mind commenting on the below?
> 
> On Mon, 9 Jan 2023, Ingo Molnar 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.
> 
> Sure, I'm fine implementing it, even in such a way so as not to cause a 
> code size/performance regression for X86_TSC configurations.  But is the 
> calculation really unpredictable enough? [...]

It's not binary: it's obviously not as good as a TSC, but my point is that 
'something cheap & variable' is clearly better than 'zero offset all the 
time'.

Thanks,

	Ingo