linux-kernel - RE: [PATCH V2 3/3] efi: Use efi_rts_workqueue to invoke EFI Runtime Services

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Date:   Thu, 8 Mar 2018 04:11:59 +0000
From:   "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@...el.com>
To:     Mark Rutland <mark.rutland@....com>
CC:     "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Lee@...rids.cambridge.arm.com" <Lee@...rids.cambridge.arm.com>,
        Chun-Yi <jlee@...e.com>, Borislav Petkov <bp@...en8.de>,
        "Luck, Tony" <tony.luck@...el.com>,
        Will Deacon <will.deacon@....com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        "Neri, Ricardo" <ricardo.neri@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Zijlstra, Peter" <peter.zijlstra@...el.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: RE: [PATCH V2 3/3] efi: Use efi_rts_workqueue to invoke EFI Runtime
 Services

05, 2018 at 03:23:10PM -0800, Sai Praneeth Prakhya wrote:
> > From: Sai Praneeth <sai.praneeth.prakhya@...el.com>
> >
> > Presently, efi_runtime_services() are executed by firmware in process
> > context. To execute efi_runtime_service(), kernel switches the page
> > directory from swapper_pgd to efi_pgd. However, efi_pgd doesn't have
> > any user space mappings. A potential issue could be, for instance, an
> > NMI interrupt (like perf) trying to profile some user data while in efi_pgd.
> 
> It might be worth pointing out that this could result in disaster (e.g.
> if the frame pointer happens to point at MMIO in the EFI runtime services
> mappings).
> 

Sorry! I didn't get it. I would like to add this point, so could you please
explain it more?

> [...]
> 
> > pstore writes could potentially be invoked in interrupt context and it
> > uses set_variable<>() and query_variable_info<>() to store logs. If we
> > invoke efi_runtime_services() through efi_rts_wq while in atomic()
> > kernel issues a warning ("scheduling wile in atomic") and prints stack
> > trace. One way to overcome this is to not make the caller process wait
> > for the worker thread to finish. This approach breaks pstore i.e. the
> > log messages aren't written to efi variables. Hence, pstore calls
> > efi_runtime_services() without using efi_rts_wq or in other words
> > efi_rts_wq will be used unconditionally for all the
> > efi_runtime_services() except set_variable<>() and
> > query_variable_info<>()
> 
> Can't NMIs still come in during this?
> 
> ... or do we assume that since something has already gone wrong, this doesn't
> matter?
>

I think they can come. AFAIK, pstore (if enabled) runs only when we crashed.
Since, we are still executing stuff to log messages and NMI's can't be masked,
there is still a possibility for NMI's to occur (please correct me if I am wrong).
But, as you said earlier, I guess it doesn't matter because anyways we are going down.
 
> Otherwise, this doesn't seem to break basic stuff on arm64 platforms. I can boot
> up, read the EFI RTC, and reboot. I ahd lockdep and KASAN enabled, I received
> no splats from either.
> 
> FWIW:
> 
> Tested-by: Mark Rutland <mark.rutland@....com [arm64]
> 
> Thanks,
> Mark.