[PATCH 1/2] KVM: Flush dirty ring resets before handling signals

[Hao Zhang <hao_zhang_kdev@163.com>]

From: Hao Zhang <zhanghao1@kylinos.cn>

KVM bails out of KVM_RESET_DIRTY_RINGS if a signal is pending, but the
dirty ring reset flow can already have consumed harvested entries before
noticing the signal.  In that case, the entries have been marked invalid
and reset_index has been advanced, but the batched GFNs may not yet have
been reprotected by kvm_reset_dirty_gfn().

This breaks the dirty ring state machine: userspace has handed the GFNs
back to KVM, but KVM can leave the corresponding pages writable.  Future
guest writes to those pages may then fail to generate new dirty-ring
entries, causing dirty tracking to miss updates.

Make the signal path break out of the scan loop instead of returning
immediately, so any pending batch is flushed before kvm_dirty_ring_reset()
returns.

Fixes: 49005a2a3d2a ("KVM: Bail from the dirty ring reset flow if a signal is pending")
Signed-off-by: Hao Zhang <zhanghao1@kylinos.cn>
---
 virt/kvm/dirty_ring.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
index 572b854edf74..451eee9d8f69 100644
--- a/virt/kvm/dirty_ring.c
+++ b/virt/kvm/dirty_ring.c
@@ -122,6 +122,7 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
 	u64 cur_offset, next_offset;
 	unsigned long mask = 0;
 	struct kvm_dirty_gfn *entry;
+	int r = 0;
 
 	/*
 	 * Ensure concurrent calls to KVM_RESET_DIRTY_RINGS are serialized,
@@ -132,8 +133,10 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
 	lockdep_assert_held(&kvm->slots_lock);
 
 	while (likely((*nr_entries_reset) < INT_MAX)) {
-		if (signal_pending(current))
-			return -EINTR;
+		if (signal_pending(current)) {
+			r = -EINTR;
+			break;
+		}
 
 		entry = &ring->dirty_gfns[ring->reset_index & (ring->size - 1)];
 
@@ -213,7 +216,7 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
 
 	trace_kvm_dirty_ring_reset(ring);
 
-	return 0;
+	return r;
 }
 
 void kvm_dirty_ring_push(struct kvm_vcpu *vcpu, u32 slot, u64 offset)

base-commit: 45c13f3f9e3bb15fd89ff2864c6f627a3b4b4229
-- 
2.15.0