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

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Date:   Mon, 21 Aug 2017 08:23:10 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        joeyli <jlee@...e.com>, Borislav Petkov <bp@...en8.de>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "Neri, Ricardo" <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3



> On Aug 21, 2017, at 7:08 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> On Mon, Aug 21, 2017 at 06:56:01AM -0700, Andy Lutomirski wrote:
>> 
>> 
>>> On Aug 21, 2017, at 3:33 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>>>> 
>>>> Using a kernel thread solves the problem for real.  Anything that
>>>> blindly accesses user memory in kernel thread context is terminally
>>>> broken no matter what.
>>> 
>>> So perf-callchain doesn't do it 'blindly', it wants either:
>>> 
>>> - user_mode(regs) true, or
>>> - task_pt_regs() set.
>>> 
>>> However I'm thinking that if the kernel thread has ->mm == &efi_mm, the
>>> EFI code running could very well have user_mode(regs) being true.
>>> 
>>> intel_pmu_pebs_fixup() OTOH 'blindly' assumes that the LBR addresses are
>>> accessible. It bails on error though. So while its careful, it does
>>> attempt to access the 'user' mapping directly. Which should also trigger
>>> with the EFI code.
>>> 
>>> And I'm not seeing anything particularly broken with either. The PEBS
>>> fixup relies on the CPU having just executed the code, and if it could
>>> fetch and execute the code, why shouldn't it be able to fetch and read?
>> 
>> There are two ways this could be a problem.  One is that u privileged
>> user apps shouldn't be able to read from EFI memory.
> 
> Ah, but only root can create per-cpu events or attach events to kernel
> threads (with sensible paranoia levels).

But this may not need to be percpu.  If a non root user can trigger, say, an EFI variable read in their own thread context, boom.

> 
>> The other is that, if EFI were to have IO memory mapped at a "user"
>> address, perf could end up reading it.
> 
> Ah, but in neither mode does perf assume much, the LBR follows branches
> the CPU took and thus we _know_ there was code there, not MMIO. And the
> stack unwind simply follows the stack up, although I suppose it could be
> 'tricked' into probing MMIO. We can certainly add an "->mm !=
> ->active_mm" escape clause to the unwind code.
> 
> Although I don't see how we're currently avoiding the same problem with
> existing userspace unwinds, userspace can equally have MMIO mapped.

But user space at least only has IO mapped to which the user program in question has rights.

> 
> But neither will use pre-existing user addresses in the efi_mm I think.