Messages in this thread | | | Date | Mon, 9 Jan 2023 11:40:40 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH v2] x86: Disable kernel stack offset randomization for !TSC |
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* 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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