Commit c64a647c authored by Alex Williamson's avatar Alex Williamson Committed by Alex Williamson
Browse files

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: 1a8a5227 ("vfio: Wait for dma-buf invalidation to complete")
Reported-by: default avatarJoonas 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: default avatarLeon Romanovsky <leon@kernel.org>
Signed-off-by: default avatarAlex Williamson <alex.williamson@nvidia.com>
Reviewed-by: default avatarJason Gunthorpe <jgg@nvidia.com>
Reviewed-by: default avatarKevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20260507143548.1018405-1-alex.williamson@nvidia.com


Signed-off-by: default avatarAlex Williamson <alex@shazbot.org>
parent 5d691905

drivers/vfio/pci/vfio_pci_dmabuf.c

+18 −18
Original line number Diff line number Diff line
@@ -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);

	}