Re: [PATCH] dma-buf: Annul dmabuf->file on file release

[Zack Rusin]

On Tue, Oct 6, 2026 at 3:15 PM Matt Evans <matt@xxxxxxxxxx> wrote:
>
> The dmabuf release path is split between file and dentry release:
> dma_buf_file_release() is called shortly before the file is freed, and
> dma_buf_release() calls an exporter's dmabuf->ops->release when the
> dentry is freed.  However, the dentry can outlive the file (for
> example, if opened with O_PATH), meaning .release might be called some
> time after the file is freed.
>
> This presents a window, when closing a DMABUF file, in which
> dmabuf->file points to freed memory yet .release op has not yet been
> called.
>
> For VFIO, if a buffer's .release has not been called it's considered
> still active and subject to move/cleanup.  If so, it attempts to
> get_file_active() on dmabuf->file: a file close with dentry held open
> will call this function with a stale pointer, a UAF.
>
> To make this pattern safe, set dmabuf->file to NULL in
> dma_buf_file_release() to reflect that the associated file is now dead
> even if the DMABUF is not yet gone.  A get_file_active() ... fput()
> sequence concurrent with a file close will only execute as one of:
>
>   - Gets the file before file_ref_put() (dmabuf->file valid)
>   - Observes dmabuf->file pointing to a file, but it's DEAD (no file)
>   - Observes dmabuf->file = NULL (no file)
>
> Originally, drivers could assume dmabuf->file was valid until .release
> was called from fops->release.  This assumption was no longer valid
> after 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to
> dentry_ops"), which moved the callback to the dentry release (by which
> point the file might have been freed).  With this commit, drivers must
> still consider that dmabuf->file could be NULL before .release.
>
> Fixes: 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to dentry_ops")
> Signed-off-by: Matt Evans <matt@xxxxxxxxxx>
> ---
>
> Hi,
>
> This issue was found (by Claude Opus 5.5) in the context of VFIO's
> DMABUF export path.  VFIO iterates live DMABUFs with a
> get_file_active()/fput() block, which now becomes safe if the file is
> closed (and memory freed!) yet DMABUF .release hasn't yet occurred.
>
> However, there are a couple of other places that directly use
> dmabuf->file and seem able to race a closing file (i.e. without holding
> the file reference)?  If this is so, they'd be a UAF today; with this
> patch that goes away, but instead of a stale pointer dmabuf->file could
> be NULL:
>
> 1. drivers/gpu/drm/vmwgfx/ttm_object.c:get_dma_buf_unless_doomed()
>
>   file_ref_get(&dmabuf->file->f_ref); on a non-refcounted DMABUF

Yes, that's a real UAF today. vmwgfx relied on the exporter's
dma_buf_ops.release running inside __fput() before file_free(), which
stopped being true with 4ab59c3c638c and we missed it.

But this patch on its own makes vmwgfx worse. In the ordinary case,
where closing the file drops the last dentry reference, a close racing
PRIME_HANDLE_TO_FD is safe today: ttm_prime_dmabuf_release() runs from
the same __fput() and blocks on prime->mutex, keeping the file
allocated, so file_ref_get() just fails on the dead refcount. With
this patch dmabuf->file is NULL in that window and we oops. So this
shouldn't go to stable without a vmwgfx fix.

For vmwgfx we'd probably need this one line change in
drivers/gpu/drm/vmwgfx/ttm_object.c to accompany this patch:
@@ -471,7 +475,7 @@ void ttm_object_device_release(struct
ttm_object_device **p_tdev)
  */
 static bool __must_check get_dma_buf_unless_doomed(struct dma_buf *dmabuf)
 {
-     return file_ref_get(&dmabuf->file->f_ref);
+     return get_file_active(&dmabuf->file) != NULL;
 }

under prime->mutex, which keeps the struct dma_buf alive. It needs the
NULL store from this patch to fix the existing UAF, since otherwise
the recheck can accept a recycled file through the unchanged stale
pointer.

> The commit message of 90ee6ed776c0 ("fs: port files to file_ref") hints
> this might be more subtle than replacing it with a get_file() variant
> (so as to accept a NULL file *).
>
> 2. drivers/gpu/drm/i915/gvt/dmabuf.c:intel_vgpu_get_dmabuf()
>
>   gvt_dbg_dpy(... file_count(dmabuf->file) ...);
>
>
> Respective vmwgfx & i915 maintainers, what is your view?
> Or, indeed, if anyone sees any other questionable uses of dmabuf->file.
>
> Thanks,
>
>
> Matt
>
>
>   drivers/dma-buf/dma-buf.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 4c9add51f9ef..726639130477 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -193,11 +193,15 @@ static void dma_buf_release(struct dentry *dentry)
>
>   static int dma_buf_file_release(struct inode *inode, struct file *file)
>   {
> +       struct dma_buf *dmabuf = file->private_data;
> +
>         if (!is_dma_buf_file(file))
>                 return -EINVAL;
>
> -       __dma_buf_list_del(file->private_data);
> +       __dma_buf_list_del(dmabuf);
>
> +       /* Must be observed by __get_file_rcu() before file_free() */
> +       smp_store_mb(dmabuf->file, NULL);

I think Alex is right and dmabuf can be NULL here. An early if
(!dmabuf) return 0" will fix that.

z

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