Re: [PATCH v8 -tip 13/26] kernel/entry: Add support for core-wide protection of kernel-mode

[Joel Fernandes]

On Mon, Nov 16, 2020 at 11:08:25AM +0100, Alexandre Chartre wrote:
[..]
> > > >    static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> > > >    					    unsigned long ti_work)
> > > >    {
> > > > @@ -175,7 +198,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> > > >    		 * enabled above.
> > > >    		 */
> > > >    		local_irq_disable_exit_to_user();
> > > > -		ti_work = READ_ONCE(current_thread_info()->flags);
> > > > +		ti_work = exit_to_user_get_work();
> > > >    	}
> > > 
> > > What happen if the task is scheduled out in exit_to_user_mode_loop? (e.g. if it has
> > > _TIF_NEED_RESCHED set). It will have call sched_core_unsafe_enter() and force siblings
> > > to wait for it. So shouldn't sched_core_unsafe_exit() be called when the task is
> > > scheduled out? (because it won't run anymore) And sched_core_unsafe_enter() when
> > > the task is scheduled back in?
> > 
> > No, when the task is scheduled out, it will in kernel mode on the task being
> > scheduled in. That task (being scheduled-in) would have already done a
> > sched_core_unsafe_enter(). When that task returns to user made, it will do a
> > sched_core_unsafe_exit(). When all tasks goto sleep, the last task that
> > enters the idle loop will do a sched_core_unsafe_exit(). Just to note: the
> > "unsafe kernel context" is per-CPU and not per-task. Does that answer your
> > question?
> 
> Ok, I think I get it: it works because when a task is scheduled out then the
> scheduler will schedule in a new tagged task (because we have core scheduling).
> So that new task should be accounted for core-wide protection the same way as
> the previous one.

Exactly!

> > > Shouldn't we be using smp_store_release() like sched_core_unsafe_exit() does?
> > > 
> > > In any case, it is worth having a comment why WRITE_ONCE() or smp_store_release() is
> > > used.
> > 
> > The smp_store_release() in exit() ensures that the write to the nesting
> > counter happens *after* all prior reads and write accesses done by this CPU
> > are seen by the spinning CPU doing the smp_load_acquire() before that
> > spinning CPU returns. I did put a comment there.
> > 
> > But, I think I don't need smp_store_release() at all here. The spin_unlock
> > that follows already has the required release semantics. I will demote it to
> > a WRITE_ONCE() in enter() as well, and add appropriate comments.
> > 
> 
> I think a WRITE_ONCE() is not even be useful here. The WRITE_ONCE() will only prevent
> some possible compiler optimization in the function wrt rq->core->core_unsafe_nest, but
> rq->core->core_unsafe_nest is just updated here, and concurrent changes are protected
> by the rq_lockp(rq) spinlock, and the memory barrier is ensured by raw_spin_unlock().
> 
> So I thing you can just do:  rq->core->core_unsafe_nest++;
> 
> And in sched_core_unsafe_exit(), you can just do:  rq->core->core_unsafe_nest = nest - 1

Hmm, I believe KCSAN will flag this as data-race though. Even though the
variable is modified under lock, it is still locklessly read in
wait_till_safe(). Though I agree that in practice it may not be that useful
because we are only checking if the variable is > 0. If its Ok with you, I
will leave it as WRITE_ONCE for now.

> Also comment in sched_core_wait_till_safe() wrt smp_load_acquire() should be updated,
> it should say:
> 
> 	/*
> 	 * Wait till the core of this HT is not in an unsafe state.
> 	 *
> 	 * Pair with raw_spin_unlock(rq_lockp(rq) in sched_core_unsafe_enter/exit()
> 	 */

Ah right, fixed. Thanks.

> > Can I add your Reviewed-by tag to below updated patch? Thanks for review!
> 
> Reviewed-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>

Will add, thanks!

 - Joel

> 
> > 
> > ---8<---
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index bd1a5b87a5e2..a36f08d74e09 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -4678,6 +4678,15 @@
> >   	sbni=		[NET] Granch SBNI12 leased line adapter
> > +	sched_core_protect_kernel=
> > +			[SCHED_CORE] Pause SMT siblings of a core running in
> > +			user mode, if at least one of the siblings of the core
> > +			is running in kernel mode. This is to guarantee that
> > +			kernel data is not leaked to tasks which are not trusted
> > +			by the kernel. A value of 0 disables protection, 1
> > +			enables protection. The default is 1. Note that protection
> > +			depends on the arch defining the _TIF_UNSAFE_RET flag.
> > +
> >   	sched_debug	[KNL] Enables verbose scheduler debug messages.
> >   	schedstats=	[KNL,X86] Enable or disable scheduled statistics.
> > diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> > index 474f29638d2c..62278c5b3b5f 100644
> > --- a/include/linux/entry-common.h
> > +++ b/include/linux/entry-common.h
> > @@ -33,6 +33,10 @@
> >   # define _TIF_PATCH_PENDING		(0)
> >   #endif
> > +#ifndef _TIF_UNSAFE_RET
> > +# define _TIF_UNSAFE_RET		(0)
> > +#endif
> > +
> >   #ifndef _TIF_UPROBE
> >   # define _TIF_UPROBE			(0)
> >   #endif
> > @@ -69,7 +73,7 @@
> >   #define EXIT_TO_USER_MODE_WORK						\
> >   	(_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE |		\
> > -	 _TIF_NEED_RESCHED | _TIF_PATCH_PENDING |			\
> > +	 _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | _TIF_UNSAFE_RET |	\
> >   	 ARCH_EXIT_TO_USER_MODE_WORK)
> >   /**
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index d38e904dd603..fe6f225bfbf9 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -2071,4 +2071,16 @@ int sched_trace_rq_nr_running(struct rq *rq);
> >   const struct cpumask *sched_trace_rd_span(struct root_domain *rd);
> > +#ifdef CONFIG_SCHED_CORE
> > +void sched_core_unsafe_enter(void);
> > +void sched_core_unsafe_exit(void);
> > +bool sched_core_wait_till_safe(unsigned long ti_check);
> > +bool sched_core_kernel_protected(void);
> > +#else
> > +#define sched_core_unsafe_enter(ignore) do { } while (0)
> > +#define sched_core_unsafe_exit(ignore) do { } while (0)
> > +#define sched_core_wait_till_safe(ignore) do { } while (0)
> > +#define sched_core_kernel_protected(ignore) do { } while (0)
> > +#endif
> > +
> >   #endif
> > diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> > index 2b8366693d5c..d5d88e735d55 100644
> > --- a/kernel/entry/common.c
> > +++ b/kernel/entry/common.c
> > @@ -28,6 +28,8 @@ static __always_inline void enter_from_user_mode(struct pt_regs *regs)
> >   	instrumentation_begin();
> >   	trace_hardirqs_off_finish();
> > +	if (_TIF_UNSAFE_RET) /* Kernel protection depends on arch defining the flag. */
> > +		sched_core_unsafe_enter();
> >   	instrumentation_end();
> >   }
> > @@ -137,6 +139,27 @@ static __always_inline void exit_to_user_mode(void)
> >   /* Workaround to allow gradual conversion of architecture code */
> >   void __weak arch_do_signal(struct pt_regs *regs) { }
> > +static unsigned long exit_to_user_get_work(void)
> > +{
> > +	unsigned long ti_work = READ_ONCE(current_thread_info()->flags);
> > +
> > +	if ((IS_ENABLED(CONFIG_SCHED_CORE) && !sched_core_kernel_protected())
> > +	    || !_TIF_UNSAFE_RET)
> > +		return ti_work;
> > +
> > +#ifdef CONFIG_SCHED_CORE
> > +	ti_work &= EXIT_TO_USER_MODE_WORK;
> > +	if ((ti_work & _TIF_UNSAFE_RET) == ti_work) {
> > +		sched_core_unsafe_exit();
> > +		if (sched_core_wait_till_safe(EXIT_TO_USER_MODE_WORK)) {
> > +			sched_core_unsafe_enter(); /* not exiting to user yet. */
> > +		}
> > +	}
> > +
> > +	return READ_ONCE(current_thread_info()->flags);
> > +#endif
> > +}
> > +
> >   static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> >   					    unsigned long ti_work)
> >   {
> > @@ -174,7 +197,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> >   		 * enabled above.
> >   		 */
> >   		local_irq_disable_exit_to_user();
> > -		ti_work = READ_ONCE(current_thread_info()->flags);
> > +		ti_work = exit_to_user_get_work();
> >   	}
> >   	/* Return the latest work state for arch_exit_to_user_mode() */
> > @@ -183,9 +206,10 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> >   static void exit_to_user_mode_prepare(struct pt_regs *regs)
> >   {
> > -	unsigned long ti_work = READ_ONCE(current_thread_info()->flags);
> > +	unsigned long ti_work;
> >   	lockdep_assert_irqs_disabled();
> > +	ti_work = exit_to_user_get_work();
> >   	if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
> >   		ti_work = exit_to_user_mode_loop(regs, ti_work);
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index fa68941998e3..429f9b8ca38e 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -76,6 +76,27 @@ __read_mostly int scheduler_running;
> >   #ifdef CONFIG_SCHED_CORE
> > +DEFINE_STATIC_KEY_TRUE(sched_core_protect_kernel);
> > +static int __init set_sched_core_protect_kernel(char *str)
> > +{
> > +	unsigned long val = 0;
> > +
> > +	if (!str)
> > +		return 0;
> > +
> > +	if (!kstrtoul(str, 0, &val) && !val)
> > +		static_branch_disable(&sched_core_protect_kernel);
> > +
> > +	return 1;
> > +}
> > +__setup("sched_core_protect_kernel=", set_sched_core_protect_kernel);
> > +
> > +/* Is the kernel protected by core scheduling? */
> > +bool sched_core_kernel_protected(void)
> > +{
> > +	return static_branch_likely(&sched_core_protect_kernel);
> > +}
> > +
> >   DEFINE_STATIC_KEY_FALSE(__sched_core_enabled);
> >   /* kernel prio, less is more */
> > @@ -4596,6 +4617,226 @@ static inline bool cookie_match(struct task_struct *a, struct task_struct *b)
> >   	return a->core_cookie == b->core_cookie;
> >   }
> > +/*
> > + * Handler to attempt to enter kernel. It does nothing because the exit to
> > + * usermode or guest mode will do the actual work (of waiting if needed).
> > + */
> > +static void sched_core_irq_work(struct irq_work *work)
> > +{
> > +	return;
> > +}
> > +
> > +static inline void init_sched_core_irq_work(struct rq *rq)
> > +{
> > +	init_irq_work(&rq->core_irq_work, sched_core_irq_work);
> > +}
> > +
> > +/*
> > + * sched_core_wait_till_safe - Pause the caller's hyperthread until the core
> > + * exits the core-wide unsafe state. Obviously the CPU calling this function
> > + * should not be responsible for the core being in the core-wide unsafe state
> > + * otherwise it will deadlock.
> > + *
> > + * @ti_check: We spin here with IRQ enabled and preempt disabled. Break out of
> > + *            the loop if TIF flags are set and notify caller about it.
> > + *
> > + * IRQs should be disabled.
> > + */
> > +bool sched_core_wait_till_safe(unsigned long ti_check)
> > +{
> > +	bool restart = false;
> > +	struct rq *rq;
> > +	int cpu;
> > +
> > +	/* We clear the thread flag only at the end, so no need to check for it. */
> > +	ti_check &= ~_TIF_UNSAFE_RET;
> > +
> > +	cpu = smp_processor_id();
> > +	rq = cpu_rq(cpu);
> > +
> > +	if (!sched_core_enabled(rq))
> > +		goto ret;
> > +
> > +	/* Down grade to allow interrupts to prevent stop_machine lockups.. */
> > +	preempt_disable();
> > +	local_irq_enable();
> > +
> > +	/*
> > +	 * Wait till the core of this HT is not in an unsafe state.
> > +	 *
> > +	 * Pair with smp_store_release() in sched_core_unsafe_exit().
> > +	 */
> > +	while (smp_load_acquire(&rq->core->core_unsafe_nest) > 0) {
> > +		cpu_relax();
> > +		if (READ_ONCE(current_thread_info()->flags) & ti_check) {
> > +			restart = true;
> > +			break;
> > +		}
> > +	}
> > +
> > +	/* Upgrade it back to the expectations of entry code. */
> > +	local_irq_disable();
> > +	preempt_enable();
> > +
> > +ret:
> > +	if (!restart)
> > +		clear_tsk_thread_flag(current, TIF_UNSAFE_RET);
> > +
> > +	return restart;
> > +}
> > +
> > +/*
> > + * Enter the core-wide IRQ state. Sibling will be paused if it is running
> > + * 'untrusted' code, until sched_core_unsafe_exit() is called. Every attempt to
> > + * avoid sending useless IPIs is made. Must be called only from hard IRQ
> > + * context.
> > + */
> > +void sched_core_unsafe_enter(void)
> > +{
> > +	const struct cpumask *smt_mask;
> > +	unsigned long flags;
> > +	struct rq *rq;
> > +	int i, cpu;
> > +
> > +	if (!static_branch_likely(&sched_core_protect_kernel))
> > +		return;
> > +
> > +	local_irq_save(flags);
> > +	cpu = smp_processor_id();
> > +	rq = cpu_rq(cpu);
> > +	if (!sched_core_enabled(rq))
> > +		goto ret;
> > +
> > +	/* Ensure that on return to user/guest, we check whether to wait. */
> > +	if (current->core_cookie)
> > +		set_tsk_thread_flag(current, TIF_UNSAFE_RET);
> > +
> > +	/* Count unsafe_enter() calls received without unsafe_exit() on this CPU. */
> > +	rq->core_this_unsafe_nest++;
> > +
> > +	/*
> > +	 * Should not nest: enter() should only pair with exit(). Both are done
> > +	 * during the first entry into kernel and the last exit from kernel.
> > +	 * Nested kernel entries (such as nested interrupts) will only trigger
> > +	 * enter() and exit() on the outer most kernel entry and exit.
> > +	 */
> > +	if (WARN_ON_ONCE(rq->core_this_unsafe_nest != 1))
> > +		goto ret;
> > +
> > +	raw_spin_lock(rq_lockp(rq));
> > +	smt_mask = cpu_smt_mask(cpu);
> > +
> > +	/*
> > +	 * Contribute this CPU's unsafe_enter() to the core-wide unsafe_enter()
> > +	 * count.  The raw_spin_unlock() release semantics pairs with the nest
> > +	 * counter's smp_load_acquire() in sched_core_wait_till_safe().
> > +	 */
> > +	WRITE_ONCE(rq->core->core_unsafe_nest, rq->core->core_unsafe_nest + 1);
> > +
> > +	if (WARN_ON_ONCE(rq->core->core_unsafe_nest == UINT_MAX))
> > +		goto unlock;
> > +
> > +	if (irq_work_is_busy(&rq->core_irq_work)) {
> > +		/*
> > +		 * Do nothing more since we are in an IPI sent from another
> > +		 * sibling to enforce safety. That sibling would have sent IPIs
> > +		 * to all of the HTs.
> > +		 */
> > +		goto unlock;
> > +	}
> > +
> > +	/*
> > +	 * If we are not the first ones on the core to enter core-wide unsafe
> > +	 * state, do nothing.
> > +	 */
> > +	if (rq->core->core_unsafe_nest > 1)
> > +		goto unlock;
> > +
> > +	/* Do nothing more if the core is not tagged. */
> > +	if (!rq->core->core_cookie)
> > +		goto unlock;
> > +
> > +	for_each_cpu(i, smt_mask) {
> > +		struct rq *srq = cpu_rq(i);
> > +
> > +		if (i == cpu || cpu_is_offline(i))
> > +			continue;
> > +
> > +		if (!srq->curr->mm || is_task_rq_idle(srq->curr))
> > +			continue;
> > +
> > +		/* Skip if HT is not running a tagged task. */
> > +		if (!srq->curr->core_cookie && !srq->core_pick)
> > +			continue;
> > +
> > +		/*
> > +		 * Force sibling into the kernel by IPI. If work was already
> > +		 * pending, no new IPIs are sent. This is Ok since the receiver
> > +		 * would already be in the kernel, or on its way to it.
> > +		 */
> > +		irq_work_queue_on(&srq->core_irq_work, i);
> > +	}
> > +unlock:
> > +	raw_spin_unlock(rq_lockp(rq));
> > +ret:
> > +	local_irq_restore(flags);
> > +}
> > +
> > +/*
> > + * Process any work need for either exiting the core-wide unsafe state, or for
> > + * waiting on this hyperthread if the core is still in this state.
> > + *
> > + * @idle: Are we called from the idle loop?
> > + */
> > +void sched_core_unsafe_exit(void)
> > +{
> > +	unsigned long flags;
> > +	unsigned int nest;
> > +	struct rq *rq;
> > +	int cpu;
> > +
> > +	if (!static_branch_likely(&sched_core_protect_kernel))
> > +		return;
> > +
> > +	local_irq_save(flags);
> > +	cpu = smp_processor_id();
> > +	rq = cpu_rq(cpu);
> > +
> > +	/* Do nothing if core-sched disabled. */
> > +	if (!sched_core_enabled(rq))
> > +		goto ret;
> > +
> > +	/*
> > +	 * Can happen when a process is forked and the first return to user
> > +	 * mode is a syscall exit. Either way, there's nothing to do.
> > +	 */
> > +	if (rq->core_this_unsafe_nest == 0)
> > +		goto ret;
> > +
> > +	rq->core_this_unsafe_nest--;
> > +
> > +	/* enter() should be paired with exit() only. */
> > +	if (WARN_ON_ONCE(rq->core_this_unsafe_nest != 0))
> > +		goto ret;
> > +
> > +	raw_spin_lock(rq_lockp(rq));
> > +	/*
> > +	 * Core-wide nesting counter can never be 0 because we are
> > +	 * still in it on this CPU.
> > +	 */
> > +	nest = rq->core->core_unsafe_nest;
> > +	WARN_ON_ONCE(!nest);
> > +
> > +	WRITE_ONCE(rq->core->core_unsafe_nest, nest - 1);
> > +	/*
> > +	 * The raw_spin_unlock release semantics pairs with the nest counter's
> > +	 * smp_load_acquire() in sched_core_wait_till_safe().
> > +	 */
> > +	raw_spin_unlock(rq_lockp(rq));
> > +ret:
> > +	local_irq_restore(flags);
> > +}
> > +
> >   // XXX fairness/fwd progress conditions
> >   /*
> >    * Returns
> > @@ -4991,6 +5232,7 @@ static inline void sched_core_cpu_starting(unsigned int cpu)
> >   			rq = cpu_rq(i);
> >   			if (rq->core && rq->core == rq)
> >   				core_rq = rq;
> > +			init_sched_core_irq_work(rq);
> >   		}
> >   		if (!core_rq)
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 001382bc67f9..20937a5b6272 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1061,6 +1061,8 @@ struct rq {
> >   	unsigned int		core_enabled;
> >   	unsigned int		core_sched_seq;
> >   	struct rb_root		core_tree;
> > +	struct irq_work		core_irq_work; /* To force HT into kernel */
> > +	unsigned int		core_this_unsafe_nest;
> >   	/* shared state */
> >   	unsigned int		core_task_seq;
> > @@ -1068,6 +1070,7 @@ struct rq {
> >   	unsigned long		core_cookie;
> >   	unsigned char		core_forceidle;
> >   	unsigned int		core_forceidle_seq;
> > +	unsigned int		core_unsafe_nest;
> >   #endif
> >   };
> >