[PATCH 7/9] vfio/pci: Support mmap() of a VFIO DMABUF

[Matt Evans]

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.

Since the main VFIO BAR mmap() is now DMABUF-aware, this path reuses
the existing vm_ops.  But, since the lifecycle of an exported DMABUF
is still decoupled from that of the device fd it came from, the device
fd might now be closed concurrent with a VMA fault.

Extra synchronisation is added to deal with the possibility of a fault
racing with the DMABUF cleanup path.  (Note that this differs to a
DMABUF implicitly created on the mmap() path, which holds ownership of
the device fd and so prevents close-during-fault scenarios in order to
maintain the same user-facing behaviour on close.)  It does this by
temporarily taking a VFIO device registration to ensure vdev remains
valid, then vdev->memory_lock can be taken.

Signed-off-by: Matt Evans <mattev@xxxxxxxx>
---
 drivers/vfio/pci/vfio_pci_core.c   | 79 ++++++++++++++++++++++++++----
 drivers/vfio/pci/vfio_pci_dmabuf.c | 28 +++++++++++
 drivers/vfio/pci/vfio_pci_priv.h   |  2 +
 3 files changed, 99 insertions(+), 10 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index 464b63585bef..cad126cf8737 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -12,6 +12,8 @@
 
 #include <linux/aperture.h>
 #include <linux/device.h>
+#include <linux/dma-buf.h>
+#include <linux/dma-resv.h>
 #include <linux/eventfd.h>
 #include <linux/file.h>
 #include <linux/interrupt.h>
@@ -1703,20 +1705,76 @@ static vm_fault_t vfio_pci_mmap_huge_fault(struct vm_fault *vmf,
 	vm_fault_t ret = VM_FAULT_SIGBUS;
 
 	/*
-	 * We can rely on the existence of both a DMABUF (priv) and
-	 * the VFIO device it was exported from (vdev).  This fault's
-	 * VMA was established using vfio_pci_core_mmap_prep_dmabuf()
-	 * which transfers ownership of the VFIO device fd to the
-	 * DMABUF, and so the VFIO device is held open because the
-	 * VMA's vm_file (DMABUF) is open.
+	 * The only thing this can rely on is that the DMABUF relating
+	 * to the VMA's vm_file exists (priv).
 	 *
-	 * Since vfio_pci_dma_buf_cleanup() cannot have happened,
-	 * vdev must be valid; we can take memory_lock.
+	 * A DMABUF for a VFIO device fd mmap() holds a reference to
+	 * the original VFIO device fd, but an explicitly-exported
+	 * DMABUF does not.  The original fd might have closed,
+	 * meaning this fault can race with
+	 * vfio_pci_dma_buf_cleanup(), meaning priv->vdev might be
+	 * NULL, and the VFIO device registration might have been
+	 * dropped.
+	 *
+	 * With the goal of taking vdev->memory_lock in a world where
+	 * vdev might not still exist:
+	 *
+	 * 1. Take the resv lock on the DMABUF:
+	 *  - If racing cleanup got in first, vdev == NULL and buffer
+	 *    is revoked; stop/exit if so.
+	 *  - If we got in first, vdev is non-NULL, accessible, and
+	 *    cleanup _has not yet put the VFIO device registration_,
+	 *    so the device refcount must be >0.
+	 *
+	 * 2. Take vfio_device registration (refcount guaranteed >0
+	 *    hereafter).
+	 *
+	 * 3. Unlock the DMABUF's resv lock:
+	 *  - A racing cleanup can now complete.
+	 *  - But, the device refcount >0, meaning the vfio_device
+	 *    (and vfio_pcie_core device vdev) have not yet been
+	 *    freed.  vdev is accessible, even if the DMABUF has been
+	 *    revoked or cleanup has happened, because
+	 *    vfio_unregister_group_dev() can't complete.
+	 *
+	 * 4. Take the vdev->memory_lock
+	 *  - Either the DMABUF is usable, or has been cleaned up.
+	 *    Whichever, it can no longer change under us.
+	 *  - Test the DMABUF revocation status again: if it was
+	 *    revoked between 1 and 4 return a SIGBUS. Otherwise,
+	 *    return a PFN.
+	 *  - It's not necessary to also take the resv lock, because
+	 *    the status/vdev can't change while memory_lock is held.
+	 *
+	 * 5. Unlock, done.
 	 */
+
+	dma_resv_lock(priv->dmabuf->resv, NULL);
 	vdev = READ_ONCE(priv->vdev);
 
+	if (READ_ONCE(priv->revoked) || !vdev) {
+		pr_debug_ratelimited("%s VA 0x%lx, pgoff 0x%lx: DMABUF revoked/cleaned up\n",
+				     __func__, vmf->address, vma->vm_pgoff);
+		dma_resv_unlock(priv->dmabuf->resv);
+		return VM_FAULT_SIGBUS;
+	}
+	/* vdev is usable */
+
+	if (!vfio_device_try_get_registration(&vdev->vdev)) {
+		/*
+		 * If vdev != NULL (above), the registration should
+		 * already be >0 and so this try_get should never
+		 * fail.
+		 */
+		dev_warn(&vdev->pdev->dev, "%s: Unexpected registration failure\n",
+			 __func__);
+		dma_resv_unlock(priv->dmabuf->resv);
+		return VM_FAULT_SIGBUS;
+	}
+	dma_resv_unlock(priv->dmabuf->resv);
+
 	scoped_guard(rwsem_read, &vdev->memory_lock) {
-		if (!priv->revoked) {
+		if (!READ_ONCE(priv->revoked)) {
 			int pres = vfio_pci_dma_buf_find_pfn(priv, vma,
 							     vmf->address,
 							     order, &pfn);
@@ -1734,6 +1792,7 @@ static vm_fault_t vfio_pci_mmap_huge_fault(struct vm_fault *vmf,
 				    vma->vm_pgoff, (unsigned int)ret);
 	}
 
+	vfio_device_put_registration(&vdev->vdev);
 	return ret;
 }
 
@@ -1742,7 +1801,7 @@ static vm_fault_t vfio_pci_mmap_page_fault(struct vm_fault *vmf)
 	return vfio_pci_mmap_huge_fault(vmf, 0);
 }
 
-static const struct vm_operations_struct vfio_pci_mmap_ops = {
+const struct vm_operations_struct vfio_pci_mmap_ops = {
 	.fault = vfio_pci_mmap_page_fault,
 #ifdef CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP
 	.huge_fault = vfio_pci_mmap_huge_fault,
diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
index 04c7733fe712..cc477f46a7d5 100644
--- a/drivers/vfio/pci/vfio_pci_dmabuf.c
+++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
@@ -27,6 +27,33 @@ static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
 
 	return 0;
 }
+
+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 (priv->revoked)
+		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_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. */
+	vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED | VM_IO | VM_PFNMAP |
+		     VM_DONTEXPAND | VM_DONTDUMP);
+	vma->vm_private_data = priv;
+	vma->vm_ops = &vfio_pci_mmap_ops;
+
+	return 0;
+}
 #endif /* CONFIG_VFIO_PCI_DMABUF */
 
 static void vfio_pci_dma_buf_done(struct kref *kref)
@@ -94,6 +121,7 @@ static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf)
 static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
 #ifdef CONFIG_VFIO_PCI_DMABUF
 	.attach = vfio_pci_dma_buf_attach,
+	.mmap = vfio_pci_dma_buf_mmap,
 #endif
 	.map_dma_buf = vfio_pci_dma_buf_map,
 	.unmap_dma_buf = vfio_pci_dma_buf_unmap,
diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
index a8edbee6ce56..f837d6c8bddc 100644
--- a/drivers/vfio/pci/vfio_pci_priv.h
+++ b/drivers/vfio/pci/vfio_pci_priv.h
@@ -37,6 +37,8 @@ struct vfio_pci_dma_buf {
 	u8 revoked : 1;
 };
 
+extern const struct vm_operations_struct vfio_pci_mmap_ops;
+
 bool vfio_pci_intx_mask(struct vfio_pci_core_device *vdev);
 void vfio_pci_intx_unmask(struct vfio_pci_core_device *vdev);
 
-- 
2.47.3