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

* 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

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