Messages in this thread |  | | Date | Tue, 21 Oct 2014 07:41:18 +0200 | From | Borislav Petkov <> | Subject | Re: [RFC 3/5] x86: Add a comment clarifying LDT context switching |
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On Thu, Oct 16, 2014 at 09:21:42AM -0700, Andy Lutomirski wrote: > I think it's the same as in the other case in switch_mm. leave_mm does > cpumask_clear_cpu(cpu, mm_cpumask(active_mm)), and, once that has > happened, modify_ldt won't send an IPI to this CPU. So, if leave_mm > runs, and then another CPU calls modify_ldt on the mm that is in lazy > mode here, it won't update our LDT register, so the LDT register and > prev->context.ldt might not match.
Ok, let me see if I can follow with an example:
We call leave_mm() on, say, cpu 3 and mm_cpumask(active_mm) has cpu 3 and 4 set. Then, on cpu 4 we call modify_ldt on that same mm and there in alloc_ldt() we have this:
if (!cpumask_equal(mm_cpumask(current->mm), cpumask_of(smp_processor_id()))) smp_call_function(flush_ldt, current->mm, 1);
and since we've cleared cpu 3 from the cpumask, we don't flush_ldt() on it and there you have the difference.
Am I close?
Thanks.
-- Regards/Gruss, Boris.
Sent from a fat crate under my desk. Formatting is fine. --
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