[RFC PATCH 2/4] rseq: Fix: Unregister rseq for CLONE_TLS

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, which makes 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 <redacted>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: "Paul E. McKenney" <redacted>
Cc: Boqun Feng <redacted>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Paul Turner <redacted>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: linux-api@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..deb4154dbf11 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_TLS set.
  */
 static inline void rseq_fork(struct task_struct *t, unsigned long clone_flags)
 {
-	if (clone_flags & CLONE_THREAD) {
+	if (clone_flags & CLONE_TLS) {
 		t->rseq = NULL;
 		t->rseq_sig = 0;
 		t->rseq_event_mask = 0;

-- 
2.17.1