[PATCH] KVM: Fix assigned device MSI-X entry setting leak

[Alex Williamson]

We need to prioritize our matching when setting MSI-X vector
entries.  Unused entries should only be used if we don't find
an exact match or else we risk duplicating entries.  This was
causing an ENOSPC return when trying to mask and unmask MSI-X
vectors based on guest MSI-X table updates.

Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>

---

The faulting sequence went something like:

Start:
[0] entry 0, vector A
[1] entry 1, vector B
[2] entry 2, vector C

Set entry 1 to 0:
[0] entry 0, vector A
[1] entry 1->1, vector B->0
[2] entry 2, vector C

Set entry 2 to 0:
[0] entry 0, vector A
[1] entry 1->2, vector 0->0 <- incorrectly matched
[2] entry 2, vector C

Set entry 2 to C:
[0] entry 0, vector A
[1] entry 2->2, vector 0->C <- incorrectly matched again
[2] entry 2, vector C

Set entry 1 to B:
[0] entry 0, vector A
[1] entry 2, vector C
[2] entry 2, vector C
-ENOSPC

Userspace will need to test for this bug (or do we want to set
a feature flag?).

 virt/kvm/assigned-dev.c |   21 +++++++++++++++------
 1 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/virt/kvm/assigned-dev.c b/virt/kvm/assigned-dev.c
index 758e3b3..798a090 100644
--- a/virt/kvm/assigned-dev.c
+++ b/virt/kvm/assigned-dev.c
@@ -728,7 +728,7 @@ msix_nr_out:
 static int kvm_vm_ioctl_set_msix_entry(struct kvm *kvm,
 				       struct kvm_assigned_msix_entry *entry)
 {
-	int r = 0, i;
+	int r = 0, i, unused = -1;
 	struct kvm_assigned_dev_kernel *adev;
 
 	mutex_lock(&kvm->lock);
@@ -741,17 +741,26 @@ static int kvm_vm_ioctl_set_msix_entry(struct kvm *kvm,
 		goto msix_entry_out;
 	}
 
-	for (i = 0; i < adev->entries_nr; i++)
-		if (adev->guest_msix_entries[i].vector == 0 ||
-		    adev->guest_msix_entries[i].entry == entry->entry) {
+	for (i = 0; i < adev->entries_nr; i++) {
+		if (unused < 0 && !adev->guest_msix_entries[i].vector)
+			unused = i;
+
+		if (adev->guest_msix_entries[i].entry == entry->entry) {
 			adev->guest_msix_entries[i].entry = entry->entry;
 			adev->guest_msix_entries[i].vector = entry->gsi;
 			adev->host_msix_entries[i].entry = entry->entry;
 			break;
 		}
+	}
+
 	if (i == adev->entries_nr) {
-		r = -ENOSPC;
-		goto msix_entry_out;
+		if (unused < 0) {
+			r = -ENOSPC;
+			goto msix_entry_out;
+		}
+		adev->guest_msix_entries[unused].entry = entry->entry;
+		adev->guest_msix_entries[unused].vector = entry->gsi;
+		adev->host_msix_entries[unused].entry = entry->entry;
 	}
 
 msix_entry_out:

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