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

From: Mathieu Desnoyers
Date: Tue Sep 10 2019 - 20:27:55 EST

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:


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@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
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;