[PATCH v3 08/15] KVM: arm64: Protect stage-2 traversal with RCU

Oliver Upton
2022-10-27T22:19:44Z

The use of RCU is necessary to safely change the stage-2 page tables in
parallel. Acquire and release the RCU read lock when traversing the page
tables.

Use the _raw() flavor of rcu_dereference when changes to the page tables
are otherwise protected from parallel software walkers (e.g. holding the
write lock).

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
arch/arm64/include/asm/kvm_pgtable.h | 41 ++++++++++++++++++++++++++++
arch/arm64/kvm/hyp/pgtable.c | 10 ++++++-
2 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index e70cf57b719e..d1859e8550df 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -37,6 +37,13 @@ static inline u64 kvm_get_parange(u64 mmfr0)

typedef u64 kvm_pte_t;

+/*
+ * RCU cannot be used in a non-kernel context such as the hyp. As such, page
+ * table walkers used in hyp do not call into RCU and instead use other
+ * synchronization mechanisms (such as a spinlock).
+ */
+#if defined(__KVM_NVHE_HYPERVISOR__) || defined(__KVM_VHE_HYPERVISOR__)
+
typedef kvm_pte_t *kvm_pteref_t;

static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared)
@@ -44,6 +51,40 @@ static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared
return pteref;
}

+static inline void kvm_pgtable_walk_begin(void) {}
+static inline void kvm_pgtable_walk_end(void) {}
+
+static inline bool kvm_pgtable_walk_lock_held(void)
+{
+ return true;
+}
+
+#else
+
+typedef kvm_pte_t __rcu *kvm_pteref_t;
+
+static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared)
+{
+ return rcu_dereference_check(pteref, shared);
+}
+
+static inline void kvm_pgtable_walk_begin(void)
+{
+ rcu_read_lock();
+}
+
+static inline void kvm_pgtable_walk_end(void)
+{
+ rcu_read_unlock();
+}
+
+static inline bool kvm_pgtable_walk_lock_held(void)
+{
+ return rcu_read_lock_held();
+}
+
+#endif
+
#define KVM_PTE_VALID BIT(0)

#define KVM_PTE_ADDR_MASK GENMASK(47, PAGE_SHIFT)
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index c4d103889a62..5bab932aeaec 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -171,6 +171,9 @@ static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data,
enum kvm_pgtable_walk_flags visit)
{
struct kvm_pgtable_walker *walker = data->walker;
+
+ /* Ensure the appropriate lock is held (e.g. RCU lock for stage-2 MMU) */
+ WARN_ON_ONCE(kvm_pgtable_walk_shared(ctx) && !kvm_pgtable_walk_lock_held());
return walker->cb(ctx, visit);
}

@@ -281,8 +284,13 @@ int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
.end = PAGE_ALIGN(walk_data.addr + size),
.walker = walker,
};
+ int r;
+
+ kvm_pgtable_walk_begin();
+ r = _kvm_pgtable_walk(pgt, &walk_data);
+ kvm_pgtable_walk_end();

- return _kvm_pgtable_walk(pgt, &walk_data);
+ return r;
}

struct leaf_walk_data {
--
2.38.1.273.g43a17bfeac-goog