Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3

[Andy Lutomirski]

On Tue, Aug 15, 2017 at 5:23 PM, Sai Praneeth Prakhya
<sai.praneeth.prakhya@xxxxxxxxx> wrote:
> On Tue, 2017-08-15 at 14:46 -0700, Andy Lutomirski wrote:
>> On Tue, Aug 15, 2017 at 12:18 PM, Sai Praneeth Prakhya
>> <sai.praneeth.prakhya@xxxxxxxxx> wrote:
>> > +/*
>> > + * Makes the calling kernel thread switch to/from efi_mm context
>> > + * Can be used from SetVirtualAddressMap() or during efi runtime calls
>> > + * (Note: This routine is heavily inspired from use_mm)
>> > + */
>> > +void efi_switch_mm(struct mm_struct *mm)
>> > +{
>> > +       struct task_struct *tsk = current;
>> > +
>> > +       task_lock(tsk);
>> > +       efi_scratch.prev_mm = tsk->active_mm;
>> > +       if (efi_scratch.prev_mm != mm) {
>> > +               mmgrab(mm);
>> > +               tsk->active_mm = mm;
>> > +       }
>> > +       switch_mm(efi_scratch.prev_mm, mm, NULL);
>> > +       task_unlock(tsk);
>> > +
>> > +       if (efi_scratch.prev_mm != mm)
>> > +               mmdrop(efi_scratch.prev_mm);
>>
>
> Thanks for the quick review Andy,
>
>> I'm confused.  You're mmdropping an mm that you are still keeping a
>> pointer to.  This is also a bit confusing in the case where you do
>> efi_switch_mm(efi_scratch.prev_mm).
>>
>
> This makes sense, I will look into it.
>
>> This whole manipulation seems fairly dangerous to me for another
>> reason -- you're taking a user thread (I think) and swapping out its
>> mm to something that the user in question should *not* have access to.
>
> We are switching to efi_mm from user mm_struct because
> EFI_RUNTIME_SERVICES like efi_set_variable()/efi_get_variable() are
> accessible only through efi_pgd. The user thread calls ioctl() which in
> turn calls efi_call() and thus efi_switch_mm(). So, I think, the user
> still does not have direct access to EFI_RUNTIME_SERVICES memory regions
> but accesses them through sys call.
>
>> What if a perf interrupt happens while you're in the alternate mm?
>
> Since we are disabling/enabling interrupts around switching, I think we
> are safe. We do these in following functions
> phys_efi_set_virtual_address_map()
> efi_thunk_set_virtual_address_map()
> efi_call_virt_pointer()

perf uses NMI, so this doesn't help.

Perhaps the sequence could look like this:

local_irq_disable();
current->active_mm = efi_mm;
switch_to();

...

switch_to(back to old mm);
current->active_mm = old mm;

and make perf know that current->active_mm != current->mm means that
user memory is off limits.