From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [PATCH for 5.3 2/3] rseq: Fix: Unregister rseq for CLONE_SETTLS Date: Mon, 16 Sep 2019 16:26:26 -0400 (EDT) Message-ID: <1809268320.7843.1568665586487.JavaMail.zimbra@efficios.com> References: <20190913151220.3105-1-mathieu.desnoyers@efficios.com> <20190913151220.3105-2-mathieu.desnoyers@efficios.com> <819646407.3304.1568470889470.JavaMail.zimbra@efficios.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <819646407.3304.1568470889470.JavaMail.zimbra@efficios.com> Sender: linux-kernel-owner@vger.kernel.org To: Thomas Gleixner , Neel Natu Cc: linux-kernel , Peter Zijlstra , paulmck , Boqun Feng , "H. Peter Anvin" , Paul Turner , Dmitry Vyukov , linux-api , stable List-Id: linux-api@vger.kernel.org ----- 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" >> Signed-off-by: Mathieu Desnoyers >> Cc: Thomas Gleixner >> Cc: Peter Zijlstra (Intel) >> Cc: "Paul E. McKenney" >> Cc: Boqun Feng >> Cc: "H . Peter Anvin" >> Cc: Paul Turner >> Cc: Dmitry Vyukov >> Cc: linux-api@vger.kernel.org >> Cc: >> --- >> 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