Messages in this thread | | | Date | Mon, 9 Jan 2023 22:53:12 +0000 (GMT) | From | "Maciej W. Rozycki" <> | Subject | Re: [PATCH v2] x86: Disable kernel stack offset randomization for !TSC |
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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? I don't feel competent enough to decide. Jason, what do you think?
Maciej
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