vfio/pci: fix dma-buf kref underflow after revoke

vfio_pci_dma_buf_move(revoked=true) and vfio_pci_dma_buf_cleanup()
ran the same drain sequence: set priv->revoked, invalidate mappings,
wait for fences, drop the registered kref, wait for completion.
When the VFIO device fd was closed after PCI_COMMAND_MEMORY had been
cleared, both ran in turn -- the second kref_put underflowed and the
subsequent wait_for_completion() blocked on a completion that the
first run had already consumed:

  refcount_t: underflow; use-after-free.
  WARNING: lib/refcount.c:28 at refcount_warn_saturate+0x59/0x90
  Call Trace:
   vfio_pci_dma_buf_cleanup+0x163/0x168 [vfio_pci_core]
   vfio_pci_core_close_device+0x67/0xe0 [vfio_pci_core]
   vfio_df_close+0x4c/0x80 [vfio]
   vfio_df_group_close+0x36/0x80 [vfio]
   vfio_device_fops_release+0x21/0x40 [vfio]
   __fput+0xe6/0x2b0
   __x64_sys_close+0x3d/0x80

Collapse the duplication: vfio_pci_dma_buf_cleanup() now delegates
the drain to vfio_pci_dma_buf_move(true), which is idempotent for
already-revoked dma-bufs.  cleanup retains only list removal and
the device registration drop; the dma_resv_lock that bracketed
those is dropped along with the in-line drain that required it,
memory_lock continues to protect them.

Re-arm the kref and the completion at the end of move()'s revoke
branch so post-revoke state matches post-creation (kref == 1,
completion ready).  This keeps cleanup's call into move() a no-op
when revoke already ran, and replaces the explicit kref_init() that
the un-revoke branch used to perform for the un-revoke -> remap
path.

Fixes: 1a8a5227f229 ("vfio: Wait for dma-buf invalidation to complete")
Reported-by: Joonas Kylmälä <joonas.kylmala@netum.fi>
Closes: https://lore.kernel.org/all/GVXPR02MB12019AA6014F27EF5D773E89BFB372@GVXPR02MB12019.eurprd02.prod.outlook.com/
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Reviewed-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Alex Williamson <alex.williamson@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20260507143548.1018405-1-alex.williamson@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>

diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
index f87fd32e4a01762cefcc393ac1ad678b33b5b4ed..fdc22e8b4656807b69517d99f245fdd5f0fb1310 100644 (file)
--- a/drivers/vfio/pci/vfio_pci_dmabuf.c
+++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
@@ -354,19 +354,18 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
                        if (revoked) {
                                kref_put(&priv->kref, vfio_pci_dma_buf_done);
                                wait_for_completion(&priv->comp);
-                       } else {
                                /*
-                                * Kref is initialize again, because when revoke
-                                * was performed the reference counter was decreased
-                                * to zero to trigger completion.
+                                * Re-arm the registered kref reference and the
+                                * completion so the post-revoke state matches the
+                                * post-creation state.  An un-revoke followed by a
+                                * new mapping needs the kref to be non-zero before
+                                * kref_get(), and vfio_pci_dma_buf_cleanup()
+                                * delegates its drain back through this revoke
+                                * path on a possibly-already-revoked dma-buf.
                                 */
                                kref_init(&priv->kref);
-                               /*
-                                * There is no need to wait as no mapping was
-                                * performed when the previous status was
-                                * priv->revoked == true.
-                                */
                                reinit_completion(&priv->comp);
+                       } else {
                                dma_resv_lock(priv->dmabuf->resv, NULL);
                                priv->revoked = false;
                                dma_resv_unlock(priv->dmabuf->resv);
@@ -382,21 +381,22 @@ void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
        struct vfio_pci_dma_buf *tmp;
 
        down_write(&vdev->memory_lock);
+
+       /*
+        * Drain any active mappings via the revoke path.  The move is
+        * idempotent for dma-bufs already in the revoked state and
+        * leaves every priv with the kref re-armed and the completion
+        * ready, so cleanup itself does not need to participate in kref
+        * bookkeeping.
+        */
+       vfio_pci_dma_buf_move(vdev, true);
+
        list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) {
                if (!get_file_active(&priv->dmabuf->file))
                        continue;
 
-               dma_resv_lock(priv->dmabuf->resv, NULL);
                list_del_init(&priv->dmabufs_elm);
                priv->vdev = NULL;
-               priv->revoked = true;
-               dma_buf_invalidate_mappings(priv->dmabuf);
-               dma_resv_wait_timeout(priv->dmabuf->resv,
-                                     DMA_RESV_USAGE_BOOKKEEP, false,
-                                     MAX_SCHEDULE_TIMEOUT);
-               dma_resv_unlock(priv->dmabuf->resv);
-               kref_put(&priv->kref, vfio_pci_dma_buf_done);
-               wait_for_completion(&priv->comp);
                vfio_device_put_registration(&vdev->vdev);
                fput(priv->dmabuf->file);
        }