linux-kernel - Re: [PATCH] x86 : Ensure X86_FLAGS

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Message-ID: <CALCETrUzRfkRYGLKxPcjuwVcg9WCyhEy1DQJjgM=JkWNUws-6w@mail.gmail.com>
Date:	Mon, 29 Sep 2014 17:11:47 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Anish Bhatt <anish@...lsio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Sebastian Lackner <sebastian@...-team.de>
Subject: Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry

On Mon, Sep 29, 2014 at 2:37 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Mon, 29 Sep 2014, Andy Lutomirski wrote:
>> On Mon, Sep 29, 2014 at 12:41 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> > On Mon, 29 Sep 2014, Andy Lutomirski wrote:
>> >> On Mon, Sep 29, 2014 at 11:59 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> >> > On Mon, 29 Sep 2014, Andy Lutomirski wrote:
>> >> >> Presumably interrupt delivery clears NT.  I haven't spotted where that's
>> >> >> documented yet.
>> >> >
>> >> > Nope, that's unrelated.
>> >>
>> >> If it weren't the case, then we'd be totally screwed.  Fortunately, it
>> >> is.  I found it: SDM Volume 3 6.12.1.2 says:
>> >>
>> >> (On calls to exception and interrupt
>> >> handlers, the processor also clears the VM, RF, and NT flags in the
>> >> EFLAGS register,
>> >> after they are saved on the stack.)
>> >
>> > Sorry, I misunderstood your question.
>> >
>> > And yes on exception and interrupt entry it is cleared. Otherwise the
>> > whole feature would not work at all ...
>> >
>> > But that's why I'm really not worried about it. While we can mask out
>> > the stupid bit easily, it does not provide any value except protecting
>> > silly userspace from rightfully raised exceptions.
>> >
>> > When I first saw that patch, I was worried about the security impact,
>> > but after staring long enough at the SDM and the code, the only way it
>> > can explode is when returning to user space. It cannot explode in the
>> > kernel.
>>
>> This is only true as long as the only use of lret from a system call
>> (or kernel thread started from a system call) is to return to
>> userspace.
>>
>> For example, __efi64_thunk uses lretq, so mixed-mode EFI doesn't
>> violate this assumption, but __efi64_thunk could just as easily have
>> used iret.
>
> And if __efi64_thunk would use iret, it would be wrong to begin with,
> really. I'd rather see it die right there.
>
>> IOW, I don't think there's any vulnerability here, but this makes me
>> nervous.
>
> I was pretty relaxed until you mentioned EFI ....

:)

I have a patch that seems to work.  It won't have any effect at all on
syscall performance (32- or 64-bit), but it slows down sysenter by 15
cycles or so.  On the other hand, if we did this, then we could
possible stop saving and restoring RFLAGS in switch_to, which might be
worthwhile, since 64-bit code ought to be more common than 32-bit
these days.

See also:
https://lkml.org/lkml/2006/9/18/161

--Andy

>
> Thanks,
>
>         tglx



-- 
Andy Lutomirski
AMA Capital Management, LLC
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