Re: [PATCH 07/14] powerpc: Add support for restartable sequences

[Michael Ellerman]

Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> writes:
> ----- On May 16, 2018, at 9:19 PM, Boqun Feng boqun.feng@xxxxxxxxx wrote:
>> On Wed, May 16, 2018 at 04:13:16PM -0400, Mathieu Desnoyers wrote:
>>> ----- On May 16, 2018, at 12:18 PM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:
>>> > On Mon, Apr 30, 2018 at 06:44:26PM -0400, Mathieu Desnoyers wrote:
>>> >> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>>> >> index c32a181a7cbb..ed21a777e8c6 100644
>>> >> --- a/arch/powerpc/Kconfig
>>> >> +++ b/arch/powerpc/Kconfig
>>> >> @@ -223,6 +223,7 @@ config PPC
>>> >>  	select HAVE_SYSCALL_TRACEPOINTS
>>> >>  	select HAVE_VIRT_CPU_ACCOUNTING
>>> >>  	select HAVE_IRQ_TIME_ACCOUNTING
>>> >> +	select HAVE_RSEQ
>>> >>  	select IRQ_DOMAIN
>>> >>  	select IRQ_FORCED_THREADING
>>> >>  	select MODULES_USE_ELF_RELA
>>> >> diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
>>> >> index 61db86ecd318..d3bb3aaaf5ac 100644
>>> >> --- a/arch/powerpc/kernel/signal.c
>>> >> +++ b/arch/powerpc/kernel/signal.c
>>> >> @@ -133,6 +133,8 @@ static void do_signal(struct task_struct *tsk)
>>> >>  	/* Re-enable the breakpoints for the signal stack */
>>> >>  	thread_change_pc(tsk, tsk->thread.regs);
>>> >>  
>>> >> +	rseq_signal_deliver(tsk->thread.regs);
>>> >> +
>>> >>  	if (is32) {
>>> >>          	if (ksig.ka.sa.sa_flags & SA_SIGINFO)
>>> >>  			ret = handle_rt_signal32(&ksig, oldset, tsk);
>>> >> @@ -164,6 +166,7 @@ void do_notify_resume(struct pt_regs *regs, unsigned long
>>> >> thread_info_flags)
>>> >>  	if (thread_info_flags & _TIF_NOTIFY_RESUME) {
>>> >>  		clear_thread_flag(TIF_NOTIFY_RESUME);
>>> >>  		tracehook_notify_resume(regs);
>>> >> +		rseq_handle_notify_resume(regs);
>>> >>  	}
>>> >>  
>>> >>  	user_enter();
>>> > 
>>> > Again no rseq_syscall().
>>> 
>>> Same question for PowerPC as for ARM:
>>> 
>>> Considering that rseq_syscall is implemented as follows:
>>> 
>>> +void rseq_syscall(struct pt_regs *regs)
>>> +{
>>> +       unsigned long ip = instruction_pointer(regs);
>>> +       struct task_struct *t = current;
>>> +       struct rseq_cs rseq_cs;
>>> +
>>> +       if (!t->rseq)
>>> +               return;
>>> +       if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) ||
>>> +           rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
>>> +               force_sig(SIGSEGV, t);
>>> +}
>>> 
>>> and that x86 calls it from syscall_return_slowpath() (which AFAIU is
>>> now used in the fast-path since KPTI), I wonder where we should call
>> 
>> So we actually detect this after the syscall takes effect, right? I
>> wonder whether this could be problematic, because "disallowing syscall"
>> in rseq areas may means the syscall won't take effect to some people, I
>> guess?
>> 
>>> this on PowerPC ?  I was under the impression that PowerPC return to
>>> userspace fast-path was not calling C code unless work flags were set,
>>> but I might be wrong.
>>> 
>> 
>> I think you're right. So we have to introduce callsite to rseq_syscall()
>> in syscall path, something like:
>> 
>> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
>> index 51695608c68b..a25734a96640 100644
>> --- a/arch/powerpc/kernel/entry_64.S
>> +++ b/arch/powerpc/kernel/entry_64.S
>> @@ -222,6 +222,9 @@ system_call_exit:
>> 	mtmsrd	r11,1
>> #endif /* CONFIG_PPC_BOOK3E */
>> 
>> +	addi    r3,r1,STACK_FRAME_OVERHEAD
>> +	bl	rseq_syscall
>> +
>> 	ld	r9,TI_FLAGS(r12)
>> 	li	r11,-MAX_ERRNO
>> 	andi.
>> 		r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK)
>> 
>> But I think it's important for us to first decide where (before or after
>> the syscall) we do the detection.
>
> As Peter said, we don't really care whether it's on syscall entry or exit, as
> long as the process gets killed when the erroneous use is detected. I think doing
> it on syscall exit is a bit easier because we can clearly access the userspace
> TLS, which AFAIU may be less straightforward on syscall entry.

Coming in to the thread late, sorry if I'm missing the point.

> We may want to add #ifdef CONFIG_DEBUG_RSEQ / #endif around the code you
> proposed above, so it's only compiled in if CONFIG_DEBUG_RSEQ=y.

That sounds good. A function call is not free even if it returns immediately.

> On the ARM leg of the email thread, Will Deacon suggests to test whether current->rseq
> is non-NULL before calling rseq_syscall(). I wonder if this added check is justified
> as the assembly level, considering that this is just a debugging option. We already do
> that check at the very beginning of rseq_syscall().

I guess it depends if this is one of those "debugging options" that's
going to end up turned on in distro kernels?

I think in that code we'd need to check paca->current->rseq, so that
wouldn't be free either.

cheers