Re: [PATCH v8 3/4] drm/virtio: implement userptr support for zero-copy memory access

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Critical] drm/virtio: privilege escalation via HINT_USERPTR_RDONLY
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commit 20ea6ca5af258a43b1dc12cab535283738613d35
Author: Honglei Huang <honghuan@amd.com>
Subject: drm/virtio: implement userptr support for zero-copy memory access

This commit adds userptr blob object support to virtio-gpu, allowing the
guest kernel to pin existing userspace mappings and use them as CREATE_BLOB
backing entries. It handles page pinning, DMA mapping if required, and
syncing the scatterlist for the device.

> diff --git a/drivers/gpu/drm/virtio/virtgpu_userptr.c b/drivers/gpu/drm/virtio/virtgpu_userptr.c
> new file mode 100644
> --- /dev/null
> +++ b/drivers/gpu/drm/virtio/virtgpu_userptr.c
[ ... ]
> +static int
> +virtio_gpu_userptr_get_pages(struct virtio_gpu_object_userptr *userptr)
> +{
> +	unsigned int flag = FOLL_LONGTERM;
> +	unsigned int num_pages, pinned = 0;
> +	int ret = 0;
[ ... ]
> +	if (!(userptr->flags & DRM_VIRTGPU_BLOB_FLAG_HINT_USERPTR_RDONLY))
> +		flag |= FOLL_WRITE;
> +
> +	do {
> +		num_pages = userptr->npages - pinned;
> +
> +		ret = pin_user_pages_fast(userptr->start + pinned * PAGE_SIZE,
> +					  num_pages, flag,
> +					  userptr->pages + pinned);

[Severity: Critical]
Does this allow unprivileged userspace to bypass guest page protections?

If a malicious guest userspace maps read-only memory (such as executable
pages like libc.so) and passes it with
DRM_VIRTGPU_BLOB_FLAG_HINT_USERPTR_RDONLY, the code omits FOLL_WRITE when
calling pin_user_pages_fast. 

The pinned physical addresses are then sent to the host. If the guest has no
vIOMMU (where virtio_gpu_use_dma_api() is false), these addresses are passed
directly to the host without DMA_TO_DEVICE IOMMU protection. The userspace
process could then submit a command stream via VIRTGPU_EXECBUFFER instructing
the host GPU to write to this resource, causing the host hypervisor to write
directly into the guest's read-only physical memory.

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Sashiko AI review · https://sashiko.dev/#/patchset/20260918055917.2133574-1-honghuan@amd.com?part=3