Messages in this thread | | | Date | Mon, 29 Sep 2014 23:37:14 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry |
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On Mon, 29 Sep 2014, Andy Lutomirski wrote: > On Mon, Sep 29, 2014 at 12:41 PM, Thomas Gleixner <tglx@linutronix.de> wrote: > > On Mon, 29 Sep 2014, Andy Lutomirski wrote: > >> On Mon, Sep 29, 2014 at 11:59 AM, Thomas Gleixner <tglx@linutronix.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 ....
Thanks,
tglx
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