linux-kernel - Re: [PATCH v4 10/10] vfio/pci: Add dma-buf export support for MMIO regions

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Message-ID: <20250929151749.2007b192.alex.williamson@redhat.com>
Date: Mon, 29 Sep 2025 15:17:49 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Leon Romanovsky <leonro@...dia.com>, Jason Gunthorpe <jgg@...dia.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Bjorn Helgaas
 <bhelgaas@...gle.com>, Christian König
 <christian.koenig@....com>, dri-devel@...ts.freedesktop.org,
 iommu@...ts.linux.dev, Jens Axboe <axboe@...nel.dk>, Joerg Roedel
 <joro@...tes.org>, kvm@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-media@...r.kernel.org, linux-mm@...ck.org, linux-pci@...r.kernel.org,
 Logan Gunthorpe <logang@...tatee.com>, Marek Szyprowski
 <m.szyprowski@...sung.com>, Robin Murphy <robin.murphy@....com>, Sumit
 Semwal <sumit.semwal@...aro.org>, Vivek Kasireddy
 <vivek.kasireddy@...el.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v4 10/10] vfio/pci: Add dma-buf export support for MMIO
 regions

On Sun, 28 Sep 2025 17:50:20 +0300
Leon Romanovsky <leon@...nel.org> wrote:
> +static int validate_dmabuf_input(struct vfio_pci_core_device *vdev,
> +				 struct vfio_device_feature_dma_buf *dma_buf,
> +				 struct vfio_region_dma_range *dma_ranges,
> +				 struct p2pdma_provider **provider)
> +{
> +	struct pci_dev *pdev = vdev->pdev;
> +	u32 bar = dma_buf->region_index;
> +	resource_size_t bar_size;
> +	u64 sum;
> +	int i;
> +
> +	if (dma_buf->flags)
> +		return -EINVAL;
> +	/*
> +	 * For PCI the region_index is the BAR number like  everything else.
> +	 */
> +	if (bar >= VFIO_PCI_ROM_REGION_INDEX)
> +		return -ENODEV;
> +
> +	*provider = pcim_p2pdma_provider(pdev, bar);
> +	if (!provider)

This needs to be IS_ERR_OR_NULL() or the function needs to settle on a
consistent error return value regardless of CONFIG_PCI_P2PDMA.

> +		return -EINVAL;
> +
> +	bar_size = pci_resource_len(pdev, bar);

We get to this feature via vfio_pci_core_ioctl_feature(), which is used
by several variant drivers, some of which mangle the BAR size exposed
to the user, ex. hisi_acc.  I'm afraid this might actually be giving
dmabuf access to a portion of the BAR that isn't exposed otherwise.

> +	for (i = 0; i < dma_buf->nr_ranges; i++) {
> +		u64 offset = dma_ranges[i].offset;
> +		u64 len = dma_ranges[i].length;
> +
> +		if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))
> +			return -EINVAL;
> +
> +		if (check_add_overflow(offset, len, &sum) || sum > bar_size)
> +			return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
> +int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> +				  struct vfio_device_feature_dma_buf __user *arg,
> +				  size_t argsz)
> +{
> +	struct vfio_device_feature_dma_buf get_dma_buf = {};
> +	struct vfio_region_dma_range *dma_ranges;
> +	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
> +	struct p2pdma_provider *provider;
> +	struct vfio_pci_dma_buf *priv;
> +	int ret;
> +
> +	ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_GET,
> +				 sizeof(get_dma_buf));
> +	if (ret != 1)
> +		return ret;
> +
> +	if (copy_from_user(&get_dma_buf, arg, sizeof(get_dma_buf)))
> +		return -EFAULT;
> +
> +	if (!get_dma_buf.nr_ranges)
> +		return -EINVAL;
> +
> +	dma_ranges = memdup_array_user(&arg->dma_ranges, get_dma_buf.nr_ranges,
> +				       sizeof(*dma_ranges));
> +	if (IS_ERR(dma_ranges))
> +		return PTR_ERR(dma_ranges);
> +
> +	ret = validate_dmabuf_input(vdev, &get_dma_buf, dma_ranges, &provider);
> +	if (ret)
> +		return ret;

goto err_free_ranges;

Thanks,
Alex