On Sat, 2017-11-11 at 10:03 -0500, Mathieu Desnoyers wrote:
> x86 has a missing core serializing instruction in migration scenarios.
>
> Given that x86-32 can return to user-space with sysexit, and x86-64
> through sysretq and sysretl, which are not core serializing, the
> following user-space self-modifiying code (JIT) scenario can occur:
Is this about load/store consistency ? In this case, don't you also
have problems with get/put_user and not just going to userspace ?
> CPU 0 CPU 1
>
> User-space self-modify code
> Preempted
> migrated ->
> scheduler selects task
> Return to user-space (iret or sysexit)
> User-space issues sync_core()
> <- migrated
> scheduler selects task
> Return to user-space (sysexit)
> jump to modified code
> Run modified code without sync_core() -> bug.
>
> This migration pattern can return to user-space through sysexit,
> sysretl, or sysretq, which are not core serializing, and therefore
> breaks sequential consistency expectations from a single-threaded
> process.
>
> Fix this issue by introducing sync_core_before_usermode(), invoked the
> first time a runqueue finishes a task switch after receiving a migrated
> thread.
>
> Architectures defining the sync_core_before_usermode() static inline
> need to define ARCH_HAS_SYNC_CORE_BEFORE_USERMODE.
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
> CC: Peter Zijlstra <pet...@infradead.org>
> CC: Andy Lutomirski <l...@kernel.org>
> CC: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> CC: Boqun Feng <boqun.f...@gmail.com>
> CC: Andrew Hunter <a...@google.com>
> CC: Maged Michael <maged.mich...@gmail.com>
> CC: Avi Kivity <a...@scylladb.com>
> CC: Benjamin Herrenschmidt <b...@kernel.crashing.org>
> CC: Paul Mackerras <pau...@samba.org>
> CC: Michael Ellerman <m...@ellerman.id.au>
> CC: Dave Watson <davejwat...@fb.com>
> CC: Thomas Gleixner <t...@linutronix.de>
> CC: Ingo Molnar <mi...@redhat.com>
> CC: "H. Peter Anvin" <h...@zytor.com>
> CC: Andrea Parri <parri.and...@gmail.com>
> CC: Russell King <li...@armlinux.org.uk>
> CC: Greg Hackmann <ghackm...@google.com>
> CC: Will Deacon <will.dea...@arm.com>
> CC: David Sehr <s...@google.com>
> CC: Linus Torvalds <torva...@linux-foundation.org>
> CC: x...@kernel.org
> CC: linux-a...@vger.kernel.org
> CC: sta...@kernel.org
>
> ---
> Changes since v1:
> - Fold patch introducing sync_core_before_usermode and the fix
> into a single patch.
> - CC sta...@kernel.org
> ---
> arch/x86/Kconfig | 1 +
> arch/x86/include/asm/processor.h | 10 ++++++++++
> include/linux/processor.h | 6 ++++++
> kernel/sched/core.c | 7 +++++++
> kernel/sched/sched.h | 1 +
> 5 files changed, 25 insertions(+)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 2fdb23313dd5..b27456f04cc6 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -62,6 +62,7 @@ config X86
> select ARCH_HAS_SG_CHAIN
> select ARCH_HAS_STRICT_KERNEL_RWX
> select ARCH_HAS_STRICT_MODULE_RWX
> + select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
> select ARCH_HAS_UBSAN_SANITIZE_ALL
> select ARCH_HAS_ZONE_DEVICE if X86_64
> select ARCH_HAVE_NMI_SAFE_CMPXCHG
> diff --git a/arch/x86/include/asm/processor.h
> b/arch/x86/include/asm/processor.h
> index bdac19ab2488..6daf70a8c81c 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -706,6 +706,16 @@ static inline void sync_core(void)
> #endif
> }
>
> +/*
> + * Ensure that a core serializing instruction is issued before returning
> + * to user-mode. x86 implements return to user-space through sysexit,
> + * sysretl, and sysretq, which are not core serializing.
> + */
> +static inline void sync_core_before_usermode(void)
> +{
> + sync_core();
> +}
> +
> extern void select_idle_routine(const struct cpuinfo_x86 *c);
> extern void amd_e400_c1e_apic_setup(void);
>
> diff --git a/include/linux/processor.h b/include/linux/processor.h
> index dbc952eec869..7d12e6fa050e 100644
> --- a/include/linux/processor.h
> +++ b/include/linux/processor.h
> @@ -68,4 +68,10 @@ do {
> \
>
> #endif
>
> +#ifndef ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
> +static inline sync_core_before_usermode(void)
> +{
> +}
> +#endif
> +
> #endif /* _LINUX_PROCESSOR_H */
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d17c5da523a0..39c0bbe8f259 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -927,6 +927,7 @@ static struct rq *move_queued_task(struct rq *rq, struct
> rq_flags *rf,
>
> rq_lock(rq, rf);
> BUG_ON(task_cpu(p) != new_cpu);
> + rq->need_sync_core = 1;
> enqueue_task(rq, p, 0);
> p->on_rq = TASK_ON_RQ_QUEUED;
> check_preempt_curr(rq, p, 0);
> @@ -2654,6 +2655,12 @@ static struct rq *finish_task_switch(struct
> task_struct *prev)
> * to use.
> */
> smp_mb__after_unlock_lock();
> +#ifdef CONFIG_SMP
> + if (unlikely(rq->need_sync_core)) {
> + sync_core_before_usermode();
> + rq->need_sync_core = 0;
> + }
> +#endif
> finish_lock_switch(rq, prev);
> finish_arch_post_lock_switch();
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 3b448ba82225..e02cc362637c 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -734,6 +734,7 @@ struct rq {
> /* For active balancing */
> int active_balance;
> int push_cpu;
> + int need_sync_core;
> struct cpu_stop_work active_balance_work;
> /* cpu of this runqueue: */
> int cpu;
