Re: [PATCH] vfio: pci: fix oops in case of vfio_msi_set_vector_signal failure

[Alex Williamson]

On Fri, 2016-01-29 at 14:43 +0000, Eric Auger wrote:
> In case vfio_msi_set_vector_signal fails we tear down everything.
> In the tear down loop we compare int j against unsigned start. Given
> the arithmetic conversion I think it is converted into an unsigned and
> becomes 0xffffffff, leading to the loop being entered again and things
> turn bad when accessing vdev->msix[vector].vector. So let's use int
> parameters instead.
>Â
> Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx>
> ---
> Âdrivers/vfio/pci/vfio_pci_intrs.c | 4 ++--
> Â1 file changed, 2 insertions(+), 2 deletions(-)
>Â
> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> index 3b3ba15..510c48d 100644
> --- a/drivers/vfio/pci/vfio_pci_intrs.c
> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> @@ -374,8 +374,8 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_device *vdev,
> Â	return 0;
> Â}
> Â
> -static int vfio_msi_set_block(struct vfio_pci_device *vdev, unsigned start,
> -			ÂÂÂÂÂÂunsigned count, int32_t *fds, bool msix)
> +static int vfio_msi_set_block(struct vfio_pci_device *vdev, int start,
> +			ÂÂÂÂÂÂint count, int32_t *fds, bool msix)
> Â{
> Â	int i, j, ret = 0;
> Â

Nice find, I don't think that's the only bug there though.ÂÂIf @start is
-1 (UINT32_MAX) and @count is 1, then @j gets set to -1 in the setup and
we hit the same index dereference problem.ÂÂWhat if we did this instead:

diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
index 3b3ba15..2ae84ad 100644
--- a/drivers/vfio/pci/vfio_pci_intrs.c
+++ b/drivers/vfio/pci/vfio_pci_intrs.c
@@ -309,14 +309,14 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_device *vdev,
Â				ÂÂÂÂÂÂint vector, int fd, bool msix)
Â{
Â	struct pci_dev *pdev = vdev->pdev;
-	int irq = msix ? vdev->msix[vector].vector : pdev->irq + vector;
-	char *name = msix ? "vfio-msix" : "vfio-msi";
Â	struct eventfd_ctx *trigger;
-	int ret;
+	int irq, ret;
Â
-	if (vector >= vdev->num_ctx)
+	if (vector < 0 || vector >= vdev->num_ctx)
Â		return -EINVAL;
Â
+	irq = msix ? vdev->msix[vector].vector : pdev->irq + vector;
+
Â	if (vdev->ctx[vector].trigger) {
Â		free_irq(irq, vdev->ctx[vector].trigger);
Â		irq_bypass_unregister_producer(&vdev->ctx[vector].producer);
@@ -328,8 +328,9 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_device *vdev,
Â	if (fd < 0)
Â		return 0;
Â
-	vdev->ctx[vector].name = kasprintf(GFP_KERNEL, "%s[%d](%s)",
-					ÂÂÂname, vector, pci_name(pdev));
+	vdev->ctx[vector].name = kasprintf(GFP_KERNEL, "vfio-msi%s[%d](%s)",
+					ÂÂÂmsix ? "x" : "", vector,
+					ÂÂÂpci_name(pdev));
Â	if (!vdev->ctx[vector].name)
Â		return -ENOMEM;
Â
@@ -379,7 +380,7 @@ static int vfio_msi_set_block(struct vfio_pci_device *vdev, unsigned start,
Â{
Â	int i, j, ret = 0;
Â
-	if (start + count > vdev->num_ctx)
+	if (start >= vdev->num_ctx || start + count > vdev->num_ctx)
Â		return -EINVAL;
Â
Â	for (i = 0, j = start; i < count && !ret; i++, j++) {
@@ -388,7 +389,7 @@ static int vfio_msi_set_block(struct vfio_pci_device *vdev, unsigned start,
Â	}
Â
Â	if (ret) {
-		for (--j; j >= start; j--)
+		for (--j; j >= 0 && j >= start; j--)
Â			vfio_msi_set_vector_signal(vdev, j, -1, msix);
Â	}
Â

So we fix the problem with vfio_msi_set_vector_signal() dereferencing
the array before it validates the index (even though it shouldn't be
able to get there anymore), and then we do a better job of verifying
start and count (comparing to num_ctx will use unsigned even though
num_ctx itself is signed) and finally explicitly test the <0 case, which
I suppose we could also do by casting start at that point (we know it's
within the bounds of a signed integer given the previous tests).
Thanks,

Alex