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 can corrupt the TLS of the parent.
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.
Here is the rationale for unregistering rseq on clone with CLONE_VM flag set:
1) CLONE_THREAD requires CLONE_SIGHAND, which requires CLONE_VM to be set. Therefore, just checking for CLONE_VM covers all CLONE_THREAD uses. There is no point in checking for both CLONE_THREAD and CLONE_VM,
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.
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: Neel Natu neelnatu@google.com Cc: linux-api@vger.kernel.org Cc: stable@vger.kernel.org # v4.18+ --- 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..d1eab0012af1 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_VM set. */ static inline void rseq_fork(struct task_struct *t, unsigned long clone_flags) { - if (clone_flags & CLONE_THREAD) { + if (clone_flags & CLONE_VM) { t->rseq = NULL; t->rseq_sig = 0; t->rseq_event_mask = 0;