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

[Mathieu Desnoyers]

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.

Thanks,

Mathieu

----- On Sep 13, 2019, at 11:12 AM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx 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@xxxxxxxxx>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: "H . Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Paul Turner <pjt@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: linux-api@xxxxxxxxxxxxxxx
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
> 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