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Groups > linux.kernel > #1712558
| From | Andy Lutomirski <luto@kernel.org> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 |
| Date | 2017-08-16 02:50 +0200 |
| Message-ID | <ueUGZ-8tc-3@gated-at.bofh.it> (permalink) |
| References | <uePHj-5pa-1@gated-at.bofh.it> <uePHj-5pa-3@gated-at.bofh.it> <ueRSN-6La-7@gated-at.bofh.it> <ueUnE-8n1-21@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
On Tue, Aug 15, 2017 at 5:23 PM, Sai Praneeth Prakhya
<sai.praneeth.prakhya@intel.com> 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@intel.com> 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.
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[PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com> - 2017-08-15 21:30 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Andy Lutomirski <luto@kernel.org> - 2017-08-15 23:50 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com> - 2017-08-16 02:30 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Andy Lutomirski <luto@kernel.org> - 2017-08-16 02:50 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-08-16 11:40 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Mark Rutland <mark.rutland@arm.com> - 2017-08-16 12:00 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Will Deacon <will.deacon@arm.com> - 2017-08-16 12:10 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Mark Rutland <mark.rutland@arm.com> - 2017-08-16 13:10 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Matt Fleming <matt@codeblueprint.co.uk> - 2017-08-16 15:00 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Andy Lutomirski <luto@kernel.org> - 2017-08-16 18:20 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Mark Rutland <mark.rutland@arm.com> - 2017-08-16 19:20 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Will Deacon <will.deacon@arm.com> - 2017-08-17 12:40 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Andy Lutomirski <luto@kernel.org> - 2017-08-17 18:00 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Peter Zijlstra <peterz@infradead.org> - 2017-08-21 12:40 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Andy Lutomirski <luto@amacapital.net> - 2017-08-21 16:00 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Peter Zijlstra <peterz@infradead.org> - 2017-08-21 16:20 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Andy Lutomirski <luto@amacapital.net> - 2017-08-21 17:30 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Peter Zijlstra <peterz@infradead.org> - 2017-08-21 18:00 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-08-21 18:10 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com> - 2017-08-24 01:00 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Peter Zijlstra <peterz@infradead.org> - 2017-08-21 19:30 +0200
Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com> - 2017-08-25 04:50 +0200
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