Re: [RFC KVM 19/27] kvm/isolation: initialize the KVM page table with core mappings

[Andy Lutomirski]

On Mon, May 13, 2019 at 8:50 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> > +     /*
> > +      * Copy the mapping for all the kernel text. We copy at the PMD
> > +      * level since the PUD is shared with the module mapping space.
> > +      */
> > +     rv = kvm_copy_mapping((void *)__START_KERNEL_map, KERNEL_IMAGE_SIZE,
> > +          PGT_LEVEL_PMD);
> > +     if (rv)
> > +             goto out_uninit_page_table;
>
> Could you double-check this?  We (I) have had some repeated confusion
> with the PTI code and kernel text vs. kernel data vs. __init.
> KERNEL_IMAGE_SIZE looks to be 512MB which is quite a bit bigger than
> kernel text.
>
> > +     /*
> > +      * Copy the mapping for cpu_entry_area and %esp fixup stacks
> > +      * (this is based on the PTI userland address space, but probably
> > +      * not needed because the KVM address space is not directly
> > +      * enterered from userspace). They can both be copied at the P4D
> > +      * level since they each have a dedicated P4D entry.
> > +      */
> > +     rv = kvm_copy_mapping((void *)CPU_ENTRY_AREA_PER_CPU, P4D_SIZE,
> > +          PGT_LEVEL_P4D);
> > +     if (rv)
> > +             goto out_uninit_page_table;
>
> cpu_entry_area is used for more than just entry from userspace.  The gdt
> mapping, for instance, is needed everywhere.  You might want to go look
> at 'struct cpu_entry_area' in some more detail.
>
> > +#ifdef CONFIG_X86_ESPFIX64
> > +     rv = kvm_copy_mapping((void *)ESPFIX_BASE_ADDR, P4D_SIZE,
> > +          PGT_LEVEL_P4D);
> > +     if (rv)
> > +             goto out_uninit_page_table;
> > +#endif
>
> Why are these mappings *needed*?  I thought we only actually used these
> fixup stacks for some crazy iret-to-userspace handling.  We're certainly
> not doing that from KVM context.
>
> Am I forgetting something?
>
> > +#ifdef CONFIG_VMAP_STACK
> > +     /*
> > +      * Interrupt stacks are vmap'ed with guard pages, so we need to
> > +      * copy mappings.
> > +      */
> > +     for_each_possible_cpu(cpu) {
> > +             stack = per_cpu(hardirq_stack_ptr, cpu);
> > +             pr_debug("IRQ Stack %px\n", stack);
> > +             if (!stack)
> > +                     continue;
> > +             rv = kvm_copy_ptes(stack - IRQ_STACK_SIZE, IRQ_STACK_SIZE);
> > +             if (rv)
> > +                     goto out_uninit_page_table;
> > +     }
> > +
> > +#endif
>
> I seem to remember that the KVM VMENTRY/VMEXIT context is very special.
>  Interrupts (and even NMIs?) are disabled.  Would it be feasible to do
> the switching in there so that we never even *get* interrupts in the KVM
> context?

That would be nicer.

Looking at this code, it occurs to me that mapping the IRQ stacks
seems questionable.  As it stands, this series switches to a normal
CR3 in some C code somewhere moderately deep in the APIC IRQ code.  By
that time, I think you may have executed traceable code, and, if that
happens, you lose.  i hate to say this, but any shenanigans like this
patch does might need to happen in the entry code *before* even
switching to the IRQ stack.  Or perhaps shortly thereafter.

We've talked about moving context tracking to C.  If we go that route,
then this KVM context mess could go there, too -- we'd have a
low-level C wrapper for each entry that would deal with getting us
ready to run normal C code.

(We need to do something about terminology.  This kvm_mm thing isn't
an mm in the normal sense.  An mm has normal kernel mappings and
varying user mappings.  For example, the PTI "userspace" page tables
aren't an mm.  And we really don't want a situation where the vmalloc
fault code runs with the "kvm_mm" mm active -- it will totally
malfunction.)