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

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Date:	Wed, 10 Aug 2016 10:30:25 -0400
From:	Chris Metcalf <cmetcalf@...lanox.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Lameter <cl@...ux.com>,
	Gilad Ben Yossef <giladb@...lanox.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Ingo Molnar" <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tejun Heo <tj@...nel.org>, Will Deacon <will.deacon@....com>,
	Rik van Riel <riel@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Francis Giraldeau <francis.giraldeau@...il.com>
Subject: Re: [PATCH v14 06/14] arch/x86: enable task isolation functionality

On 8/10/2016 3:52 AM, Andy Lutomirski wrote:
> On Aug 9, 2016 11:30 PM, "Chris Metcalf" <cmetcalf@...lanox.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.

Thanks!

>>   #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.

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com