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Commit a2b7861b authored by Boqun Feng's avatar Boqun Feng Committed by Radim Krčmář
Browse files

kvm/x86: Avoid async PF preempting the kernel incorrectly 



Currently, in PREEMPT_COUNT=n kernel, kvm_async_pf_task_wait() could call
schedule() to reschedule in some cases.  This could result in
accidentally ending the current RCU read-side critical section early,
causing random memory corruption in the guest, or otherwise preempting
the currently running task inside between preempt_disable and
preempt_enable.

The difficulty to handle this well is because we don't know whether an
async PF delivered in a preemptible section or RCU read-side critical section
for PREEMPT_COUNT=n, since preempt_disable()/enable() and rcu_read_lock/unlock()
are both no-ops in that case.

To cure this, we treat any async PF interrupting a kernel context as one
that cannot be preempted, preventing kvm_async_pf_task_wait() from choosing
the schedule() path in that case.

To do so, a second parameter for kvm_async_pf_task_wait() is introduced,
so that we know whether it's called from a context interrupting the
kernel, and the parameter is set properly in all the callsites.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: default avatarBoqun Feng <boqun.feng@gmail.com>
Signed-off-by: default avatarRadim Krčmář <rkrcmar@redhat.com>
parent 2fb1e946

arch/x86/include/asm/kvm_para.h

+2 −2
Original line number Diff line number Diff line
@@ -88,7 +88,7 @@ static inline long kvm_hypercall4(unsigned int nr, unsigned long p1, 
bool kvm_para_available(void);

unsigned int kvm_arch_para_features(void);

void __init kvm_guest_init(void);

void kvm_async_pf_task_wait(u32 token);

void kvm_async_pf_task_wait(u32 token, int interrupt_kernel);

void kvm_async_pf_task_wake(u32 token);

u32 kvm_read_and_reset_pf_reason(void);

extern void kvm_disable_steal_time(void);
@@ -103,7 +103,7 @@ static inline void kvm_spinlock_init(void) 


#else /* CONFIG_KVM_GUEST */

#define kvm_guest_init() do {} while (0)

#define kvm_async_pf_task_wait(T) do {} while(0)

#define kvm_async_pf_task_wait(T, I) do {} while(0)

#define kvm_async_pf_task_wake(T) do {} while(0)



static inline bool kvm_para_available(void)

arch/x86/kernel/kvm.c

+10 −4
Original line number Diff line number Diff line
@@ -117,7 +117,11 @@ static struct kvm_task_sleep_node *_find_apf_task(struct kvm_task_sleep_head *b, 
	return NULL;

}



void kvm_async_pf_task_wait(u32 token)

/*

 * @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)

{

	u32 key = hash_32(token, KVM_TASK_SLEEP_HASHBITS);

	struct kvm_task_sleep_head *b = &async_pf_sleepers[key];
@@ -140,8 +144,10 @@ void kvm_async_pf_task_wait(u32 token) 


	n.token = token;

	n.cpu = smp_processor_id();

	n.halted = is_idle_task(current) || preempt_count() > 1 ||

		   rcu_preempt_depth();

	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);
@@ -269,7 +275,7 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code) 
	case KVM_PV_REASON_PAGE_NOT_PRESENT:

		/* page is swapped out by the host. */

		prev_state = exception_enter();

		kvm_async_pf_task_wait((u32)read_cr2());

		kvm_async_pf_task_wait((u32)read_cr2(), !user_mode(regs));

		exception_exit(prev_state);

		break;

	case KVM_PV_REASON_PAGE_READY:

arch/x86/kvm/mmu.c

+1 −1
Original line number Diff line number Diff line
@@ -3837,7 +3837,7 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code, 
	case KVM_PV_REASON_PAGE_NOT_PRESENT:

		vcpu->arch.apf.host_apf_reason = 0;

		local_irq_disable();

		kvm_async_pf_task_wait(fault_address);

		kvm_async_pf_task_wait(fault_address, 0);

		local_irq_enable();

		break;

	case KVM_PV_REASON_PAGE_READY: