Date:   Wed, 1 May 2019 13:25:56 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [RESEND PATCH v6 08/12] x86/fsgsbase/64: Use the per-CPU base as GSBASE at the paranoid_entry



> On May 1, 2019, at 1:21 PM, Bae, Chang Seok <chang.seok.bae@...el.com> wrote:
> 
> 
>>> On May 1, 2019, at 11:01, Bae, Chang Seok <chang.seok.bae@...el.com> wrote:
>>> 
>>> On May 1, 2019, at 10:40, Andy Lutomirski <luto@...nel.org> wrote:
>>> 
>>> On Wed, May 1, 2019 at 6:52 AM Bae, Chang Seok <chang.seok.bae@...el.com> wrote:
>>>> 
>>>> 
>>>>> On Apr 5, 2019, at 06:50, Andy Lutomirski <luto@...capital.net> wrote:
>>>>> 
>>>>> Furthermore, if you folks even want me to review this series, the ptrace tests need to be in place.  On inspection of the current code (after the debacle a few releases back), it appears the SETREGSET’s effect depends on the current values in the registers — it does not actually seem to reliably load the whole state. So my confidence will be greatly increased if your series first adds a test that detects that bug (and fails!), then fixes the bug in a tiny little patch, then adds FSGSBASE, and keeps the test working.
>>>>> 
>>>> 
>>>> I think I need to understand the issue. Appreciate if you can elaborate a little bit.
>>>> 
>>> 
>>> This patch series gives a particular behavior to PTRACE_SETREGS and
>>> PTRACE_POKEUSER.  There should be a test case that validates that
>>> behavior, including testing the weird cases where gs != 0 and gsbase
>>> contains unusual values.  Some existing tests might be pretty close to
>>> doing what's needed.
>>> 
>>> Beyond that, the current putreg() code does this:
>>> 
>>>  case offsetof(struct user_regs_struct,gs_base):
>>>      /*
>>>       * Exactly the same here as the %fs handling above.
>>>       */
>>>      if (value >= TASK_SIZE_MAX)
>>>          return -EIO;
>>>      if (child->thread.gsbase != value)
>>>          return do_arch_prctl_64(child, ARCH_SET_GS, value);
>>>      return 0;
>>> 
>>> and do_arch_prctl_64(), in turn, does this:
>>> 
>>>  case ARCH_SET_GS: {
>>>      if (unlikely(arg2 >= TASK_SIZE_MAX))
>>>          return -EPERM;
>>> 
>>>      preempt_disable();
>>>      /*
>>>       * ARCH_SET_GS has always overwritten the index
>>>       * and the base. Zero is the most sensible value
>>>       * to put in the index, and is the only value that
>>>       * makes any sense if FSGSBASE is unavailable.
>>>       */
>>>      if (task == current) {
>>>       [not used for ptrace]
>>>      } else {
>>>          task->thread.gsindex = 0;
>>>          x86_gsbase_write_task(task, arg2);
>>>      }
>>> 
>>>      ...
>>> 
>>> So writing the value that was already there to gsbase via putreg()
>>> does nothing, but writing a *different* value implicitly clears gs,
>>> but writing a different value will clear gs.
>>> 
>>> This behavior is, AFAICT, complete nonsense.  It happens to work
>>> because usually gdb writes the same value back, and, in any case, gs
>>> comes *after* gsbase in user_regs_struct, so gs gets replaced anyway.
>>> But I think that this behavior should be fixed up and probably tested.
>>> Certainly the behavior should *not* be the same on a fsgsbase kernel,
>>> and and the fsgsbase behavior definitely needs a selftest.
>> 
>> Okay, got the point; now crystal clear.
>> 
>> I have my own test case for that though, need to find a very simple and
>> acceptable solution.
>> 
> 
> One solution that I recall, HPA once suggested, is:
>    Write registers in a reverse order from user_regs_struct, for SETREGS
> 
> Assuming these for clarification, first:
>    * old and new index != 0
>    * taking GS as an example though, should be the same with FS
> 
> Then, interesting cases would be something like these, without FSGSBASE:
>    Case (a), when index only changed to (new index):
>        (Then, the result after SETREGS would be)
>        GS = (new index), GSBASE = the base fetched from (new index)
>    Case (b), when base only changed to (new base):
>    Case (c), when both are changed:
>        GS = 0, GSBASE = (new base)
> 
> Now, with FSGSBASE:
>    Case (a):
>        GS = (new index), GSBASE = (old base)
>    Case (b):
>        GS = (old index), GSBASE = (new base)
>    Case (c):
>        GS = (new index), GSBASE = (new base)
> 
> As a reference, today's kernel behavior, without FSGSBASE:
>    Case (a):
>        GS = (new index), GSBASE = the base fetched from (new index)
>    Case (b):
>        GS = (old index), GSBASE = (old base)
>    Case (c):
>        GS = (new index), GSBASE = the base fetched from (new index)
> 
> Now, with that reverse ordering and taking that "GSBASE is important" [1],
> it looks like to be working in terms of its base value:
>    Case (b) and (c) will behave the same as with FSGSBASE
>    Case (a) still differs between w/ and w/o FSGSBASE.
>        Well, I'd say this bit comes from the 'new model' vs. the 'leagcy
>        model'. So, then okay with that. Any thoughts?
> 
> 
> 

This seems more complicated than needed.  How about we just remove all the magic and make putreg on the base registers never change the selector.

As far as I can tell, the only downside is that, on a non-FSGSBASE kernel, setting only the base if the selector already has a nonzero value won’t work, but I would be quite surprised if this breaks anything.

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