Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry
From: Andy Lutomirski
Date: Mon Sep 29 2014 - 20:12:57 EST
On Mon, Sep 29, 2014 at 2:37 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Mon, 29 Sep 2014, Andy Lutomirski wrote:
>> 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 3 184.108.40.206 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
>> 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
> 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
AMA Capital Management, LLC
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