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