KVM: Warn if mark_page_dirty() is called without an active vCPU

The various kvm_write_guest() and mark_page_dirty() functions must only
ever be called in the context of an active vCPU, because if dirty ring
tracking is enabled it may simply oops when kvm_get_running_vcpu()
returns NULL for the vcpu and then kvm_dirty_ring_get() dereferences it.

This oops was reported by "butt3rflyh4ck" <butterflyhuangxx@gmail.com> in
https://lore.kernel.org/kvm/CAFcO6XOmoS7EacN_n6v4Txk7xL7iqRa2gABg3F7E3Naf5uG94g@mail.gmail.com/

That actual bug will be fixed under separate cover but this warning
should help to prevent new ones from being added.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20211210163625.2886-2-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/include/linux/kvm_dirty_ring.h b/include/linux/kvm_dirty_ring.h
index 4da8d4a..906f899 100644 (file)
--- a/include/linux/kvm_dirty_ring.h
+++ b/include/linux/kvm_dirty_ring.h
@@ -43,11 +43,6 @@ static inline int kvm_dirty_ring_alloc(struct kvm_dirty_ring *ring,
        return 0;
 }
 
-static inline struct kvm_dirty_ring *kvm_dirty_ring_get(struct kvm *kvm)
-{
-       return NULL;
-}
-
 static inline int kvm_dirty_ring_reset(struct kvm *kvm,
                                       struct kvm_dirty_ring *ring)
 {
@@ -78,7 +73,6 @@ static inline bool kvm_dirty_ring_soft_full(struct kvm_dirty_ring *ring)
 
 u32 kvm_dirty_ring_get_rsvd_entries(void);
 int kvm_dirty_ring_alloc(struct kvm_dirty_ring *ring, int index, u32 size);
-struct kvm_dirty_ring *kvm_dirty_ring_get(struct kvm *kvm);
 
 /*
  * called with kvm->slots_lock held, returns the number of

diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
index 88f4683..8e98747 100644 (file)
--- a/virt/kvm/dirty_ring.c
+++ b/virt/kvm/dirty_ring.c
@@ -36,15 +36,6 @@ static bool kvm_dirty_ring_full(struct kvm_dirty_ring *ring)
        return kvm_dirty_ring_used(ring) >= ring->size;
 }
 
-struct kvm_dirty_ring *kvm_dirty_ring_get(struct kvm *kvm)
-{
-       struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
-
-       WARN_ON_ONCE(vcpu->kvm != kvm);
-
-       return &vcpu->dirty_ring;
-}
-
 static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
 {
        struct kvm_memory_slot *memslot;

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b0f7e6e..af5b442 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3155,12 +3155,17 @@ void mark_page_dirty_in_slot(struct kvm *kvm,
                             const struct kvm_memory_slot *memslot,
                             gfn_t gfn)
 {
+       struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
+
+       if (WARN_ON_ONCE(!vcpu) || WARN_ON_ONCE(vcpu->kvm != kvm))
+               return;
+
        if (memslot && kvm_slot_dirty_track_enabled(memslot)) {
                unsigned long rel_gfn = gfn - memslot->base_gfn;
                u32 slot = (memslot->as_id << 16) | memslot->id;
 
                if (kvm->dirty_ring_size)
-                       kvm_dirty_ring_push(kvm_dirty_ring_get(kvm),
+                       kvm_dirty_ring_push(&vcpu->dirty_ring,
                                            slot, rel_gfn);
                else
                        set_bit_le(rel_gfn, memslot->dirty_bitmap);