MIME-Version: 1.0
In-Reply-To: <20140605155307.GD16642@pd.tnic>
References: <tip-baa916f39b50ad91661534652110df40396acda0@git.kernel.org>
 <538F9AFA.5050806@zytor.com> <20140604224920.GB4126@pd.tnic>
 <538FB775.8070405@amacapital.net> <20140605071805.GA16647@pd.tnic>
 <CAL01qpuj6jvH4XLvhHAszbqmSPXfW=VOdv+TqMwBuDnArL=ofw@mail.gmail.com>
 <20140605090206.GA16642@pd.tnic> <CALCETrU2TZhxOZyCvBd1nLDiU4ceiSx_Sd2bsDUCX5arOFidBg@mail.gmail.com>
 <20140605155307.GD16642@pd.tnic>
From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 5 Jun 2014 09:01:02 -0700
Message-ID: <CALCETrVqqgExhuAYawh_7tc8-Nm8GGiMxmtw25mLFoxru-odZg@mail.gmail.com>
Subject: Re: [tip:x86/efi] x86/efi: Check for unsafe dealing with FPU state in
 irq ctxt
To: Borislav Petkov <bp@alien8.de>
Cc: Matt Fleming <matt.fleming@intel.com>, "H. Peter Anvin" <hpa@zytor.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "linux-tip-commits@vger.kernel.org" 
	<linux-tip-commits@vger.kernel.org>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org

On Thu, Jun 5, 2014 at 8:53 AM, Borislav Petkov <bp@alien8.de> wrote:
> On Thu, Jun 05, 2014 at 08:44:20AM -0700, Andy Lutomirski wrote:
>> Are there weird contexts from which EFI calls can happen? It looks
>> like the current code isn't necessarily safe in things that aren't
>> normal process context but aren't interrupts either (e.g. debug traps,
>> #GP, etc).
>
> The efi-pstore thing registers as a kmsg dumper which can be run in NMI
> context and efi can be called there.

NMI might be okay.  I haven't checked.

>
>> I wonder if it would make sense at some point to maintain an explicit
>> stack of kernel entries. There doesn't seem to be a reliable way to
>> answer the question of "what context am I in" from C code right now.
>
> So that you can ask int ctxt = what_context_Im_in() and then that
> context can go and change right underneath you. :-)
>

It has to change back, though.  Completely unrealistic and useless example:

int ctxt = what_context_im_in();

set_up_the_fpu(ctxt);

// kprobe fires and changes the context
// kprobe does something
// kprobe changes the context back

use the FPU.  Life is good.

put_back_the_fpu(ctxt);

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/