linux-kernel - Re: FSGSBASE ABI considerations

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Date:   Mon, 7 Aug 2017 09:14:43 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Brian Gerst <brgerst@...il.com>, Stas Sergeev <stsp@...t.ru>
Subject: Re: FSGSBASE ABI considerations

On Jul 31, 2017, at 5:23 PM, Bae, Chang Seok <chang.seok.bae@...el.com> wrote:

>> On an FSGSBASE-enabled system, I think we need to provide deterministic, documented, tested behavior.  I can think of three plausible choices:
>> 1a. modify_ldt() immediately updates FSBASE and GSBASE all threads that reference the modified selector.
>> 1b. modify_ldt() immediatley updates FSBASE and GSBASE on all threads that reference the LDT.
>> 2. modify_ldt() leaves FSBASE and GSBASE alone on all threads.
>> (2) is trivial to implement, whereas (1a) and (1b) are a bit nasty to implement when FSGSBASE is on.
>
>> The tricky bit is that 32-bit kernels can't do (2), so, if we want modify_ldt() to behave the same on 32-bit and 64-bit kernels, we're stuck with (1).
>
> While implementing (1) is still unclear for context switch, here is one idea for (1b):
> - thread struct has new entry for ldt pointer that last seen
> - modify_ldt happens
> - ldtr upated for active threads via IPI
> - for inactive threads being scheduled in, ldtr updated before __switch_to
> - in __switch_to, read ldtr by sldt and compare the new ldt pointer
>    sldt is ucode that likely takes only a couple cycles
> - mostly matched given modify_ldt is rare
> - unmatched, don't write gsbase if gs indicating LDT

That won't be reliable -- LDTR could change more than once and be
reused between context switches.  If we went this route, I think we'd
put a u64 version in ldt_struct.  We'd also need to audit and fix up
every access to thread.fs/gsbase.