A VFIO DMABUF can export a subset of a BAR to userspace by fd; add support for mmap() of this fd. This provides another route for a process to map BARs, except one where the process can only map a specific subset of a BAR represented by the exported DMABUF.
mmap() support enables userspace driver designs that safely delegate access to BAR sub-ranges to other client processes by sharing a DMABUF fd, without having to share the (omnipotent) VFIO device fd with them.
The mmap callback installs vm_ops callbacks for .fault and .huge_fault; they find a PFN by searching the DMABUF's physical ranges. That is, DMABUFs with multiple ranges are supported for mmap().
Signed-off-by: Matt Evans mattev@meta.com --- drivers/vfio/pci/vfio_pci_dmabuf.c | 219 +++++++++++++++++++++++++++++ 1 file changed, 219 insertions(+)
diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c index 46ab64fbeb19..bebb496bd0f2 100644 --- a/drivers/vfio/pci/vfio_pci_dmabuf.c +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c @@ -85,6 +85,209 @@ static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf) kfree(priv); }
+static int vfio_pci_dma_buf_find_pfn(struct device *dev, + struct vfio_pci_dma_buf *vpdmabuf, + struct vm_area_struct *vma, + unsigned long address, + unsigned int order, + unsigned long *out_pfn) +{ + /* + * Given a VMA (start, end, pgoffs) and a fault address, + * search phys_vec[] to find the range representing the + * address's offset into the VMA (and so a PFN). + * + * The phys_vec ranges represent contiguous spans of VAs + * upwards from the buffer offset 0; the actual PFNs might be + * in any order, overlap/alias, etc. Calculate an offset of + * the desired page given VMA start/pgoff and address, then + * search upwards from 0 to find which span contains it. + * + * On success, a valid PFN for a page sized by 'order' is + * returned into out_pfn. + * + * Failure occurs if: + * - The page would cross the edge of the VMA + * - The page isn't entirely contained within a range + * - We find a range, but the final PFN isn't aligned to the + * requested order. + * + * (Upon failure, the caller is expected to try again with a + * smaller order; the tests above will always succeed for + * order=0 as the limit case.) + * + * It's suboptimal if DMABUFs are created with neigbouring + * ranges that are physically contiguous, since hugepages + * can't straddle range boundaries. (The construction of the + * ranges vector should merge such ranges.) + */ + + unsigned long rounded_page_addr = address & ~((PAGE_SIZE << order) - 1); + unsigned long rounded_page_end = rounded_page_addr + (PAGE_SIZE << order); + unsigned long buf_page_offset; + unsigned long buf_offset = 0; + unsigned int i; + + if (rounded_page_addr < vma->vm_start || rounded_page_end > vma->vm_end) + return -EAGAIN; + + if (unlikely(check_add_overflow(rounded_page_addr - vma->vm_start, + vma->vm_pgoff << PAGE_SHIFT, &buf_page_offset))) + return -EFAULT; + + for (i = 0; i < vpdmabuf->nr_ranges; i++) { + unsigned long range_len = vpdmabuf->phys_vec[i].len; + unsigned long range_start = vpdmabuf->phys_vec[i].paddr; + + if (buf_page_offset >= buf_offset && + buf_page_offset + (PAGE_SIZE << order) <= buf_offset + range_len) { + /* + * The faulting page is wholly contained + * within the span represented by the range. + * Validate PFN alignment for the order: + */ + unsigned long pfn = (range_start >> PAGE_SHIFT) + + ((buf_page_offset - buf_offset) >> PAGE_SHIFT); + + if (IS_ALIGNED(pfn, 1 << order)) { + *out_pfn = pfn; + return 0; + } + /* Retry with smaller order */ + return -EAGAIN; + } + buf_offset += range_len; + } + + /* + * If we get here, the address fell outside of the span + * represented by the (concatenated) ranges. This can + * never happen because vfio_pci_dma_buf_mmap() checks that + * the VMA is <= the total size of the ranges. + * + * But if it does, force SIGBUS for the access, and warn. + */ + WARN_ONCE(1, "No range for addr 0x%lx, order %d: VMA 0x%lx-0x%lx pgoff 0x%lx, %d ranges, size 0x%lx\n", + address, order, vma->vm_start, vma->vm_end, vma->vm_pgoff, + vpdmabuf->nr_ranges, vpdmabuf->size); + + return -EFAULT; +} + +static vm_fault_t vfio_pci_dma_buf_mmap_huge_fault(struct vm_fault *vmf, + unsigned int order) +{ + struct vm_area_struct *vma = vmf->vma; + struct vfio_pci_dma_buf *priv = vma->vm_private_data; + struct vfio_pci_core_device *vdev; + unsigned long pfn; + vm_fault_t ret = VM_FAULT_FALLBACK; + + vdev = READ_ONCE(priv->vdev); + + /* + * A fault for an existing mmap might occur after + * vfio_pci_dma_buf_cleanup() has revoked and destroyed the + * vdev's DMABUFs, and annulled vdev. After creation, vdev is + * only ever written in cleanup. + */ + if (!vdev) + return VM_FAULT_SIGBUS; + + int r = vfio_pci_dma_buf_find_pfn(&vdev->pdev->dev, priv, vma, + vmf->address, order, &pfn); + + if (r == 0) { + scoped_guard(rwsem_read, &vdev->memory_lock) { + /* Deal with the possibility of a fault racing + * with vfio_pci_dma_buf_move() revoking and + * then unmapping the buffer. The + * revocation/unmap and status change occurs + * whilst holding memory_lock. + */ + if (priv->revoked) + ret = VM_FAULT_SIGBUS; + else + ret = vfio_pci_vmf_insert_pfn(vdev, vmf, pfn, order); + } + } else if (r != -EAGAIN) { + ret = VM_FAULT_SIGBUS; + } + + dev_dbg_ratelimited(&vdev->pdev->dev, + "%s(order = %d) PFN 0x%lx, VA 0x%lx, pgoff 0x%lx: 0x%x\n", + __func__, order, pfn, vmf->address, vma->vm_pgoff, (unsigned int)ret); + + return ret; +} + +static vm_fault_t vfio_pci_dma_buf_mmap_page_fault(struct vm_fault *vmf) +{ + return vfio_pci_dma_buf_mmap_huge_fault(vmf, 0); +} + +static const struct vm_operations_struct vfio_pci_dma_buf_mmap_ops = { + .fault = vfio_pci_dma_buf_mmap_page_fault, +#ifdef CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP + .huge_fault = vfio_pci_dma_buf_mmap_huge_fault, +#endif +}; + +static bool vfio_pci_dma_buf_is_mappable(struct dma_buf *dmabuf) +{ + struct vfio_pci_dma_buf *priv = dmabuf->priv; + + /* + * Sanity checks at mmap() time; alignment has already been + * asserted by validate_dmabuf_input(). + * + * Although the revoked state is transient, refuse to map a + * revoked buffer to flag early that something odd is going + * on: for example, users should not be mmap()ing a buffer + * that's being moved [by a user-triggered activity]. + */ + if (priv->revoked) + return false; + + return true; +} + +/* + * Similar to vfio_pci_core_mmap() for a regular VFIO device fd, but + * differs by pre-checks performed and ultimately the vm_ops installed. + */ +static int vfio_pci_dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma) +{ + struct vfio_pci_dma_buf *priv = dmabuf->priv; + u64 req_len, req_start; + + if (!vfio_pci_dma_buf_is_mappable(dmabuf)) + return -ENODEV; + if ((vma->vm_flags & VM_SHARED) == 0) + return -EINVAL; + + req_len = vma->vm_end - vma->vm_start; + req_start = vma->vm_pgoff << PAGE_SHIFT; + + if (req_start + req_len > priv->size) + return -EINVAL; + + vma->vm_private_data = priv; + vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot); + vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot); + + /* + * See comments in vfio_pci_core_mmap() re VM_ALLOW_ANY_UNCACHED. + * + * FIXME: get mapping attributes from dmabuf? + */ + vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED | VM_IO | VM_PFNMAP | + VM_DONTEXPAND | VM_DONTDUMP); + vma->vm_ops = &vfio_pci_dma_buf_mmap_ops; + + return 0; +} + static const struct dma_buf_ops vfio_pci_dmabuf_ops = { .pin = vfio_pci_dma_buf_pin, .unpin = vfio_pci_dma_buf_unpin, @@ -92,6 +295,7 @@ static const struct dma_buf_ops vfio_pci_dmabuf_ops = { .map_dma_buf = vfio_pci_dma_buf_map, .unmap_dma_buf = vfio_pci_dma_buf_unmap, .release = vfio_pci_dma_buf_release, + .mmap = vfio_pci_dma_buf_mmap, };
/* @@ -335,6 +539,11 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked) struct vfio_pci_dma_buf *tmp;
lockdep_assert_held_write(&vdev->memory_lock); + /* + * Holding memory_lock ensures a racing + * vfio_pci_dma_buf_mmap_*_fault() observes priv->revoked + * properly. + */
list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) { if (!get_file_active(&priv->dmabuf->file)) @@ -345,6 +554,14 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked) priv->revoked = revoked; dma_buf_move_notify(priv->dmabuf); dma_resv_unlock(priv->dmabuf->resv); + + /* + * Unmap any possible userspace mappings for a + * now-revoked DMABUF: + */ + if (revoked) + unmap_mapping_range(priv->dmabuf->file->f_mapping, + 0, priv->size, 1); } fput(priv->dmabuf->file); } @@ -366,6 +583,8 @@ void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev) priv->revoked = true; dma_buf_move_notify(priv->dmabuf); dma_resv_unlock(priv->dmabuf->resv); + unmap_mapping_range(priv->dmabuf->file->f_mapping, + 0, priv->size, 1); vfio_device_put_registration(&vdev->vdev); fput(priv->dmabuf->file); }