Re: [patch 2/2] x86/kvm: Sanitize kvm_async_pf_task_wait()

["Paul E. McKenney" <paulmck@kernel.org>]

On Sat, Mar 07, 2020 at 12:42:06AM +0100, Thomas Gleixner wrote:
> While working on the entry consolidation I stumbled over the KVM async page
> fault handler and kvm_async_pf_task_wait() in particular. It took me a
> while to realize that the randomly sprinkled around rcu_irq_enter()/exit()
> invocations are just cargo cult programming. Several patches "fixed" RCU
> splats by curing the symptoms without noticing that the code is flawed 
> from a design perspective.
> 
> The main problem is that this async injection is not based on a proper
> handshake mechanism and only respects the minimal requirement, i.e. the
> guest is not in a state where it has interrupts disabled.
> 
> Aside of that the actual code is a convoluted one fits it all swiss army
> knife. It is invoked from different places with different RCU constraints:
> 
>   1) Host side:
> 
>      vcpu_enter_guest()
>        kvm_x86_ops->handle_exit()
>          kvm_handle_page_fault()
>            kvm_async_pf_task_wait()
> 
>      The invocation happens from fully preemptible context.
> 
>   2) Guest side:
> 
>      The async page fault interrupted:
> 
>          a) user space
> 
> 	 b) preemptible kernel code which is not in a RCU read side
> 	    critical section
> 
>      	 c) non-preemtible kernel code or a RCU read side critical section
> 	    or kernel code with CONFIG_PREEMPTION=n which allows not to
> 	    differentiate between #2b and #2c.
> 
> RCU is watching for:
> 
>   #1  The vCPU exited and current is definitely not the idle task
> 
>   #2a The #PF entry code on the guest went through enter_from_user_mode()
>       which reactivates RCU
> 
>   #2b There is no preemptible, interrupts enabled code in the kernel
>       which can run with RCU looking away. (The idle task is always
>       non preemptible).
> 
> I.e. all schedulable states (#1, #2a, #2b) do not need any of this RCU
> voodoo at all.
> 
> In #2c RCU is eventually not watching, but as that state cannot schedule
> anyway there is no point to worry about it so it has to invoke
> rcu_irq_enter() before running that code. This can be optimized, but this
> will be done as an extra step in course of the entry code consolidation
> work.

In other words, any needed rcu_irq_enter() and rcu_irq_exit() are added
in one of the entry-code consolidation patches, and patch below depends
on that patch, correct?

							Thanx, Paul

> So the proper solution for this is to:
> 
>   - Split kvm_async_pf_task_wait() into schedule and halt based waiting
>     interfaces which share the enqueueing code.
> 
>   - Add comments (condensed form of this changelog) to spare others the
>     time waste and pain of reverse engineering all of this with the help of
>     uncomprehensible changelogs and code history.
> 
>   - Invoke kvm_async_pf_task_wait_schedule() from kvm_handle_page_fault(),
>     user mode and schedulable kernel side async page faults (#1, #2a, #2b)
> 
>   - Invoke kvm_async_pf_task_wait_halt() for the non schedulable kernel
>     case (#2c).
> 
>     For this case also remove the rcu_irq_exit()/enter() pair around the
>     halt as it is just a pointless exercise:
> 
>        - vCPUs can VMEXIT at any any random point and can be scheduled out
>          for an arbitrary amount of time by the host and this is not any
>          different except that it voluntary triggers the exit via halt.
> 
>        - The interrupted context could have RCU watching already. So the
> 	 rcu_irq_exit() before the halt is not gaining anything aside of
> 	 confusing the reader. Claiming that this might prevent RCU stalls
> 	 is just an illusion.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  arch/x86/include/asm/kvm_para.h |    2 
>  arch/x86/kernel/kvm.c           |  156 ++++++++++++++++++++++++++++------------
>  arch/x86/kvm/mmu/mmu.c          |    2 
>  3 files changed, 115 insertions(+), 45 deletions(-)
> 
> --- a/arch/x86/include/asm/kvm_para.h
> +++ b/arch/x86/include/asm/kvm_para.h
> @@ -88,7 +88,7 @@ static inline long kvm_hypercall4(unsign
>  bool kvm_para_available(void);
>  unsigned int kvm_arch_para_features(void);
>  unsigned int kvm_arch_para_hints(void);
> -void kvm_async_pf_task_wait(u32 token, int interrupt_kernel);
> +void kvm_async_pf_task_wait_schedule(u32 token);
>  void kvm_async_pf_task_wake(u32 token);
>  u32 kvm_read_and_reset_pf_reason(void);
>  void kvm_disable_steal_time(void);
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -75,7 +75,7 @@ struct kvm_task_sleep_node {
>  	struct swait_queue_head wq;
>  	u32 token;
>  	int cpu;
> -	bool halted;
> +	bool use_halt;
>  };
>  
>  static struct kvm_task_sleep_head {
> @@ -98,75 +98,145 @@ static struct kvm_task_sleep_node *_find
>  	return NULL;
>  }
>  
> -/*
> - * @interrupt_kernel: Is this called from a routine which interrupts the kernel
> - * 		      (other than user space)?
> - */
> -void kvm_async_pf_task_wait(u32 token, int interrupt_kernel)
> +static bool kvm_async_pf_queue_task(u32 token, bool use_halt,
> +				    struct kvm_task_sleep_node *n)
>  {
>  	u32 key = hash_32(token, KVM_TASK_SLEEP_HASHBITS);
>  	struct kvm_task_sleep_head *b = &async_pf_sleepers[key];
> -	struct kvm_task_sleep_node n, *e;
> -	DECLARE_SWAITQUEUE(wait);
> -
> -	rcu_irq_enter();
> +	struct kvm_task_sleep_node *e;
>  
>  	raw_spin_lock(&b->lock);
>  	e = _find_apf_task(b, token);
>  	if (e) {
>  		/* dummy entry exist -> wake up was delivered ahead of PF */
>  		hlist_del(&e->link);
> -		kfree(e);
>  		raw_spin_unlock(&b->lock);
> +		kfree(e);
> +		return false;
> +	}
>  
> -		rcu_irq_exit();
> +	n->token = token;
> +	n->cpu = smp_processor_id();
> +	n->use_halt = use_halt;
> +	init_swait_queue_head(&n->wq);
> +	hlist_add_head(&n->link, &b->list);
> +	raw_spin_unlock(&b->lock);
> +	return true;
> +}
> +
> +/*
> + * kvm_async_pf_task_wait_schedule - Wait for pagefault to be handled
> + * @token:	Token to identify the sleep node entry
> + *
> + * Invoked from the async pagefault handling code or from the VM exit page
> + * fault handler. In both cases RCU is watching.
> + */
> +void kvm_async_pf_task_wait_schedule(u32 token)
> +{
> +	struct kvm_task_sleep_node n;
> +	DECLARE_SWAITQUEUE(wait);
> +
> +	lockdep_assert_irqs_disabled();
> +
> +	if (!kvm_async_pf_queue_task(token, false, &n))
>  		return;
> +
> +	for (;;) {
> +		prepare_to_swait_exclusive(&n.wq, &wait, TASK_UNINTERRUPTIBLE);
> +		if (hlist_unhashed(&n.link))
> +			break;
> +
> +		local_irq_enable();
> +		schedule();
> +		local_irq_disable();
>  	}
> +	finish_swait(&n.wq, &wait);
> +}
> +EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait_schedule);
>  
> -	n.token = token;
> -	n.cpu = smp_processor_id();
> -	n.halted = is_idle_task(current) ||
> -		   (IS_ENABLED(CONFIG_PREEMPT_COUNT)
> -		    ? preempt_count() > 1 || rcu_preempt_depth()
> -		    : interrupt_kernel);
> -	init_swait_queue_head(&n.wq);
> -	hlist_add_head(&n.link, &b->list);
> -	raw_spin_unlock(&b->lock);
> +/*
> + * Invoked from the async page fault handler.
> + */
> +static void kvm_async_pf_task_wait_halt(u32 token)
> +{
> +	struct kvm_task_sleep_node n;
> +
> +	if (!kvm_async_pf_queue_task(token, true, &n))
> +		return;
>  
>  	for (;;) {
> -		if (!n.halted)
> -			prepare_to_swait_exclusive(&n.wq, &wait, TASK_UNINTERRUPTIBLE);
>  		if (hlist_unhashed(&n.link))
>  			break;
> +		/*
> +		 * No point in doing anything about RCU here. Any RCU read
> +		 * side critical section or RCU watching section can be
> +		 * interrupted by VMEXITs and the host is free to keep the
> +		 * vCPU scheduled out as long as it sees fit. This is not
> +		 * any different just because of the halt induced voluntary
> +		 * VMEXIT.
> +		 *
> +		 * Also the async page fault could have interrupted any RCU
> +		 * watching context, so invoking rcu_irq_exit()/enter()
> +		 * around this is not gaining anything.
> +		 */
> +		native_safe_halt();
> +		local_irq_disable();
> +	}
> +}
>  
> -		rcu_irq_exit();
> +/* Invoked from the async page fault handler */
> +static void kvm_async_pf_task_wait(u32 token, bool usermode)
> +{
> +	bool can_schedule;
>  
> -		if (!n.halted) {
> -			local_irq_enable();
> -			schedule();
> -			local_irq_disable();
> -		} else {
> -			/*
> -			 * We cannot reschedule. So halt.
> -			 */
> -			native_safe_halt();
> -			local_irq_disable();
> -		}
> +	/*
> +	 * No need to check whether interrupts were disabled because the
> +	 * host will (hopefully) only inject an async page fault into
> +	 * interrupt enabled regions.
> +	 *
> +	 * If CONFIG_PREEMPTION is enabled then check whether the code
> +	 * which triggered the page fault is preemptible. This covers user
> +	 * mode as well because preempt_count() is obviously 0 there.
> +	 *
> +	 * The check for rcu_preempt_depth() is also required because
> +	 * voluntary scheduling inside a rcu read locked section is not
> +	 * allowed.
> +	 *
> +	 * The idle task is already covered by this because idle always
> +	 * has a preempt count > 0.
> +	 *
> +	 * If CONFIG_PREEMPTION is disabled only allow scheduling when
> +	 * coming from user mode as there is no indication whether the
> +	 * context which triggered the page fault could schedule or not.
> +	 */
> +	if (IS_ENABLED(CONFIG_PREEMPTION))
> +		can_schedule = preempt_count() + rcu_preempt_depth() == 0;
> +	else
> +		can_schedule = usermode;
>  
> +	/*
> +	 * If the kernel context is allowed to schedule then RCU is
> +	 * watching because no preemptible code in the kernel is inside RCU
> +	 * idle state. So it can be treated like user mode. User mode is
> +	 * safe because the #PF entry invoked enter_from_user_mode().
> +	 *
> +	 * For the non schedulable case invoke rcu_irq_enter() for
> +	 * now. This will be moved out to the pagefault entry code later
> +	 * and only invoked when really needed.
> +	 */
> +	if (can_schedule) {
> +		kvm_async_pf_task_wait_schedule(token);
> +	} else {
>  		rcu_irq_enter();
> +		kvm_async_pf_task_wait_halt(token);
> +		rcu_irq_exit();
>  	}
> -	if (!n.halted)
> -		finish_swait(&n.wq, &wait);
> -
> -	rcu_irq_exit();
> -	return;
>  }
> -EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait);
>  
>  static void apf_task_wake_one(struct kvm_task_sleep_node *n)
>  {
>  	hlist_del_init(&n->link);
> -	if (n->halted)
> +	if (n->use_halt)
>  		smp_send_reschedule(n->cpu);
>  	else if (swq_has_sleeper(&n->wq))
>  		swake_up_one(&n->wq);
> @@ -255,7 +325,7 @@ bool __kvm_handle_async_pf(struct pt_reg
>  		return false;
>  	case KVM_PV_REASON_PAGE_NOT_PRESENT:
>  		/* page is swapped out by the host. */
> -		kvm_async_pf_task_wait(token, !user_mode(regs));
> +		kvm_async_pf_task_wait(token, user_mode(regs));
>  		return true;
>  	case KVM_PV_REASON_PAGE_READY:
>  		rcu_irq_enter();
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4205,7 +4205,7 @@ int kvm_handle_page_fault(struct kvm_vcp
>  	case KVM_PV_REASON_PAGE_NOT_PRESENT:
>  		vcpu->arch.apf.host_apf_reason = 0;
>  		local_irq_disable();
> -		kvm_async_pf_task_wait(fault_address, 0);
> +		kvm_async_pf_task_wait_schedule(fault_address);
>  		local_irq_enable();
>  		break;
>  	case KVM_PV_REASON_PAGE_READY:
>