Re: [PATCH v14 06/14] arch/x86: enable task isolation functionality

[Andy Lutomirski]

On Wed, Aug 10, 2016 at 12:40 PM, Chris Metcalf <cmetcalf@xxxxxxxxxxxx> wrote:
> On 8/10/2016 3:17 PM, Andy Lutomirski wrote:
>>
>> On Aug 10, 2016 5:30 PM, "Chris Metcalf" <cmetcalf@xxxxxxxxxxxx> wrote:
>>>
>>> On 8/10/2016 3:52 AM, Andy Lutomirski wrote:
>>>>
>>>> On Aug 9, 2016 11:30 PM, "Chris Metcalf" <cmetcalf@xxxxxxxxxxxx> wrote:
>>>> @@ -91,6 +92,15 @@ static long syscall_trace_enter(struct pt_regs *regs)
>>>>           if (emulated)
>>>>                   return -1L;
>>>>
>>>> +       /* In isolation mode, we may prevent the syscall from running.
>>>> */
>>>> +       if (work & _TIF_TASK_ISOLATION) {
>>>> +               if (task_isolation_syscall(regs->orig_ax) == -1) {
>>>> +                       regs->orig_ax = -1;
>>>> +                       return 0;
>>>> +               }
>>>> +               work &= ~_TIF_TASK_ISOLATION;
>>>> +       }
>>>> +
>>>> What is this?  It's not mentioned in the changelog.  It seems
>>>> nonsensical to me.  If nothing else, you forgot to update regs->ax,
>>>> but I don't even know what you're trying to do.
>>>
>>>
>>> It's mentioned in the changelog as "Fixes a bug in x86
>>> syscall_trace_enter()
>>> [seen by Francis Giraldeau]."  To be fair, I didn't hear back from
>>> Francis, and
>>> you're right, this doesn't look like it makes any sense now.  (I've added
>>> him
>>> to the cc's on this email; for this series I had just put him on the
>>> cover letter.)
>>>
>>> I modeled this code on a snippet from the old two-phase syscall entry
>>> work:
>>>
>>>                 if (ret == SECCOMP_PHASE1_SKIP) {
>>>                         regs->orig_ax = -1;
>>>                         ret = 0;
>>>                 }
>>>
>>> You got rid of this during the 4.7-rc series, but my code above was at
>>> least
>>> plausibly valid until then :-)
>>>
>>> Regardless, I assume that the right thing for that condition to do now
>>> when
>>> it triggers is to set regs->ax = -ENOSYS and return -1L?  I'll update the
>>> git repository with that in any case.
>>
>> regs->ax will already be -ENOSYS unless something changed it
>
>
> Right, I see that now in entry_SYSCALL_64_after_swapgs.  Good.
>
>> but I'm
>> not sure what this code is trying to do.  Is the idea that
>> task_isolation_syscall might enqueue a signal and you want to deliver
>> it without processing the syscall?  If so, a comment would be nice.
>> You could even WARN_ON(!signal_pending()).
>
>
> If you are in task isolation mode (and you haven't also requested NOSIG),
> then attempting a system call fails, and you get a signal delivered.  You
> convinced me that failing the syscall was the thing to do back here:
>
> https://lkml.kernel.org/r/CALCETrUrc_LJyLJLHefSDYagCrNqqzKuknr6uLgVXnPW8PmZKw@xxxxxxxxxxxxxx

Indeed.  Then please add the comment any maybe the WARN_ON.

>
>>>>>    #ifdef CONFIG_SECCOMP
>>>>>           /*
>>>>>            * Do seccomp after ptrace, to catch any tracer changes.
>>>>> @@ -136,7 +146,7 @@ static long syscall_trace_enter(struct pt_regs
>>>>> *regs)
>>>>>
>>>>>    #define EXIT_TO_USERMODE_LOOP_FLAGS                            \
>>>>>           (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE |   \
>>>>> -        _TIF_NEED_RESCHED | _TIF_USER_RETURN_NOTIFY)
>>>>> +        _TIF_NEED_RESCHED | _TIF_USER_RETURN_NOTIFY |
>>>>> _TIF_TASK_ISOLATION)
>>>>>
>>>> Where are you updating the conditions to force use of the slow path?
>>>> (That's _TIF_ALLWORK_MASK.)
>>>
>>>
>>> Whenever _TIF_TASK_ISOLATION is set, _TIF_NOHZ is also set.
>>
>> OK, but why not decouple it a bit and add it to the mask?  I keep
>> meaning to add a BUILD_BUG_ON checking for bits in
>> EXIT_TO_USERMODE_LOOP_FLAGS that aren't in the appropriate slow path
>> masks.
>
>
> That does seem reasonable; I'll make the change.
>
>
> --
> Chris Metcalf, Mellanox Technologies
> http://www.mellanox.com
>



-- 
Andy Lutomirski
AMA Capital Management, LLC