Messages in this thread | | | Date | Mon, 29 Sep 2014 12:43:32 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry |
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On 09/29/2014 12:41 PM, Thomas Gleixner wrote: >> >> 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. > > So in IA-32e it creates a #GP otherwise it falls over the return to > NULL (TSS.back_link). So what? >
How about "it's a bug, but it's not (necessarily) a security issue?"
I think we should mask the bit anyway.
-hpa
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