Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry

[Andy Lutomirski]

On Mon, Sep 29, 2014 at 12:41 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Mon, 29 Sep 2014, Andy Lutomirski wrote:
>> On Mon, Sep 29, 2014 at 11:59 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.

IOW, I don't think there's any vulnerability here, but this makes me nervous.

--Andy
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