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SubjectRe: [PATCH v14 06/14] arch/x86: enable task isolation functionality
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On 8/10/2016 3:17 PM, Andy Lutomirski wrote:
> On Aug 10, 2016 5:30 PM, "Chris Metcalf" <cmetcalf@mellanox.com> wrote:
>> On 8/10/2016 3:52 AM, Andy Lutomirski wrote:
>>> On Aug 9, 2016 11:30 PM, "Chris Metcalf" <cmetcalf@mellanox.com> 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@mail.gmail.com

>>>> #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

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