Subject: Re: FSGSBASE ABI considerations
To: Andy Lutomirski <luto@kernel.org>
Cc: "Bae, Chang Seok" <chang.seok.bae@intel.com>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bpetkov@suse.de>, Brian Gerst <brgerst@gmail.com>,
        Bart Oldeman <bartoldeman@users.sourceforge.net>
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From: Stas Sergeev <stsp@list.ru>
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Date: Tue, 8 Aug 2017 00:32:29 +0300
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07.08.2017 19:20, Andy Lutomirski пишет:
>> I think
>> this is the half-step. It clearly shows that you don't want
>> such state to ever exist, but why not to go a step further
>> and just make the bases to be reset not only by any
>> unrelated modify_ldt() call, but always on schedule?
>> You can state that using wrgsbase on non-zero selector
>> is invalid, reset it to LDT state and maybe send a signal
>> to the program so that it knows it did something wrong.
>> This may sound too rough, but I really don't see how it
>> differs from resetting all LDT bases on some unrelated
>> modify_ldt() that was done for read, not write.
>> Or you may want to reset selector to 0 rather than
>> base to LDT.
> Windows does something sort of like this (I think), but I don't like
> this solution.  I fully expect that someone will write a program that
> does:
>
> old = rdgsbase();
> wrgsbase(new);
> call_very_fast_function();
> wrgsbase(old);
>
> This will work if GS == 0, which is fine.  The problem is that it will
> *also* work if GS != 0 with very high probability, especially if this
> code sequence is right after some operation that sleeps.  And then
> we'll get random crashes with very low probability, depending on where
> the scheduler hits.
So, as Linus already pointed, if the fixup is to
zero out the selector, then this will still work fine.


>> I am far from the kernel development so my thoughts
>> may be naive, but IMHO you should just disallow this
>> by some means (like by doing a fixup on schedule() and
>> sending a signal). No one will suffer, people will just
>> write 0 to segreg first. Note that such a problem can
>> be provoked by the fact that the sighandler does not
>> reset the segregs to their default values, and someone
>> may simply forget to reset it to 0. You need to remind
>> him to do so rather than to invent the tricky code to
>> do something theoretically correct.
> I would *love* to disallow it.  The problem is that I don't believe it
> to be possible in a way that doesn't cause more problems than it
> solves.
I wonder if sending a signal (after doing a fixup)
is too much of a punishment?

> I'm trying to avoid a situation where we implement that policy and the
> interaction with modify_ldt() becomes very strange.
IMHO if you do the fixup on schedule (like setting
the selector to zero), then the interaction with
modify_ldt() is completely avoided, i.e. modify_ldt()
should then never special-case the threads that
did wrgsbase. So if something inconsistent comes
out, then it was likely there already without wrgsbase.