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Subject[PATCH] kvm/x86: Avoid async PF to end RCU read-side critical section early in PREEMPT=n kernel
Date
Currently, in PREEMPT=n kernel, kvm_async_pf_task_wait() could call
schedule() to reschedule in some cases, which could result in
accidentally ending the current RCU read-side critical section early.
And this could end up with random memory corruption in the guest.

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

To cure this, we treat any async PF interrupting a kernel context as one
delivered in a RCU read-side critical section, and we don't allow
kvm_async_pf_task_wait() to choose schedule path in that case for
PREEMPT_COUNT=n kernel, because that will introduce unvolunteerly
context switches and break the assumption for RCU to work properly.

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 we set that parameter 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>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
arch/x86/include/asm/kvm_para.h | 4 ++--
arch/x86/kernel/kvm.c | 14 ++++++++++----
arch/x86/kvm/mmu.c | 2 +-
3 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index bc62e7cbf1b1..59ad3d132353 100644
--- 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(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)
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index e675704fa6f7..8bb9594d0761 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -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:
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index eca30c1eb1d9..106d4a029a8a 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -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:
--
2.14.1
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