Re: [PATCH for 5.3 2/3] rseq: Fix: Unregister rseq for CLONE_SETTLS

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Sep 14, 2019, at 10:21 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> There is an ongoing discussion on the choice of flag we want to care
> about here. Therefore, please don't pull this patch until we reach an
> agreement.

Following discussion with Neel Natu (Google) and Paul Turner (Google),
I plan to modify this patch, and unregister RSEQ on clone CLONE_VM for the
following reasons:

1) CLONE_THREAD requires CLONE_SIGHAND, which requires CLONE_VM to be
   set. Therefore, just checking for CLONE_VM covers all CLONE_THREAD uses,

2) There is the possibility of an unlikely scenario where CLONE_SETTLS is used
   without CLONE_VM. In order to be an issue, it would require that the rseq
   TLS is in a shared memory area.

   I do not plan on adding CLONE_SETTLS to the set of clone flags which
   unregister RSEQ, because it would require that we also unregister RSEQ
   on set_thread_area(2) and arch_prctl(2) ARCH_SET_FS for completeness.
   So rather than doing a partial solution, it appears better to let user-space
   explicitly perform rseq unregistration across clone if needed in scenarios
   where CLONE_VM is not set.

Thoughts ?

Thanks,

Mathieu


> 
> Thanks,
> 
> Mathieu
> 
> ----- On Sep 13, 2019, at 11:12 AM, Mathieu Desnoyers
> mathieu.desnoyers@efficios.com wrote:
> 
>> It has been reported by Google that rseq is not behaving properly
>> with respect to clone when CLONE_VM is used without CLONE_THREAD.
>> It keeps the prior thread's rseq TLS registered when the TLS of the
>> thread has moved, so the kernel deals with the wrong TLS.
>> 
>> The approach of clearing the per task-struct rseq registration
>> on clone with CLONE_THREAD flag is incomplete. It does not cover
>> the use-case of clone with CLONE_VM set, but without CLONE_THREAD.
>> 
>> Looking more closely at each of the clone flags:
>> 
>> - CLONE_THREAD,
>> - CLONE_VM,
>> - CLONE_SETTLS.
>> 
>> It appears that the flag we really want to track is CLONE_SETTLS, which
>> moves the location of the TLS for the child, making the rseq
>> registration point to the wrong TLS.
>> 
>> Suggested-by: "H . Peter Anvin" <hpa@zytor.com>
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
>> Cc: Boqun Feng <boqun.feng@gmail.com>
>> Cc: "H . Peter Anvin" <hpa@zytor.com>
>> Cc: Paul Turner <pjt@google.com>
>> Cc: Dmitry Vyukov <dvyukov@google.com>
>> Cc: linux-api@vger.kernel.org
>> Cc: <stable@vger.kernel.org>
>> ---
>> include/linux/sched.h | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 9f51932bd543..76bf55b5cccf 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1919,11 +1919,11 @@ static inline void rseq_migrate(struct task_struct *t)
>> 
>> /*
>>  * If parent process has a registered restartable sequences area, the
>> - * child inherits. Only applies when forking a process, not a thread.
>> + * child inherits. Unregister rseq for a clone with CLONE_SETTLS set.
>>  */
>> static inline void rseq_fork(struct task_struct *t, unsigned long clone_flags)
>> {
>> -	if (clone_flags & CLONE_THREAD) {
>> +	if (clone_flags & CLONE_SETTLS) {
>> 		t->rseq = NULL;
>> 		t->rseq_sig = 0;
>> 		t->rseq_event_mask = 0;
>> --
>> 2.17.1
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com