Re: [PATCH 4/7] x86: Add support for rd/wr fs/gs base

Andy Lutomirski
2014-05-01T22:06:31Z

On Thu, May 1, 2014 at 2:58 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 05/01/2014 02:15 PM, Andi Kleen wrote:
>>> If usergs == kernelgs, then ebx will always be 1 and we'll never end
>>> up in paranoid_userspace.
>>
>> You may miss a reschedule in this obscure case. It shouldn't really
>> happen because loading a kernel pointer is not useful for user space.
>>
>> Doesn't seem like a real issue to me.
>>
>> We only happen need to handle it to avoid crashing.
>>
>
> No, it would be a rootable security hole, not just a crash.
>
>>> Alternatively, what if the paranoid entry checked whether we're coming
>>> from userspace at the very beginning and, if so, just jumped to the
>>> non-paranoid entry?
>>
>> That would work, but I doubt it would be worth it.
>
> If that would solve the problem it is simple enough, but the tricky part
> is when we end up in a "crack" where we are in kernel mode with the user GS.
>
> I haven't looked through the flows (I'm at LCE so I have limited screen
> bandwidth) to see how that would be handled in this case, but in the
> general paranoid case it comes down to the fact that in this particular
> subcase we don't necessarily know exactly how many SWAPGS are between us
> and userspace after we IRET.

The current code looks like it will never try to reschedule on
paranoid exit unless it came from user *CS*, in which case there
shouldn't be any weird gs issues. Given that the current code won't
reschedule even on a paranoid entry that hits during interruptable
kernel code, I find it unlikely that this code is important. You
probably know more about its history and significance than I do.

What happens when ftrace or perf tries to wake a task from a debug
interrupt or NMI?

--Andy