[PATCH v3 0/2] dma-buf: fix fd leak when copy_to_user() fails after fd_install()

[Baineng Shou]

Several drivers call dma_buf_fd() — which internally calls fd_install()
— before copy_to_user() returns the fd number to userspace.  If
copy_to_user() fails, the fd is already published in the caller's fd
table but the ioctl returns an error, so userspace never learns the fd
number.  Worse, the window between fd_install() and copy_to_user()
allows other threads to observe and manipulate the fd (dup, close,
SCM_RIGHTS), making any "close it on the failure path" fix unsafe.

The fix is to split the allocation into three steps: reserve an fd with
get_unused_fd_flags() (not yet visible to other threads), do
copy_to_user(), and only then publish the fd with fd_install() via the
new dma_buf_fd_install() helper.  On copy_to_user() failure,
put_unused_fd() + dma_buf_put() cleanly unwind with no user-visible
side effects.

Patch 1 introduces dma_buf_fd_install() in dma-buf.c (wrapping
fd_install() together with the DMA_BUF_TRACE call to preserve export
tracing) and applies the fix to dma-heap.

Patch 2 applies the same fix to fastrpc, which even had a comment
acknowledging the problem could not be fixed before.

v1: https://lore.kernel.org/dri-devel/20260703080922.1838362-1-shoubaineng@xxxxxxxxx/
v2: https://lore.kernel.org/dri-devel/20260710105430.3059661-1-shoubaineng@xxxxxxxxx/

Changes in v3:
 - Split into two patches (dma-heap + fastrpc separately)
 - Add dma_buf_fd_install() to preserve trace_dma_buf_fd tracepoint
   (spotted by T.J. Mercier and sashiko-bot on v2)
 - Add fastrpc fix using the new helper (suggested by T.J. Mercier)

Baineng Shou (2):
  dma-buf: dma-heap: don't publish fd before copy_to_user() succeeds
  misc: fastrpc: don't publish fd before copy_to_user() succeeds

 drivers/dma-buf/dma-buf.c  | 20 ++++++++++
 drivers/dma-buf/dma-heap.c | 80 +++++++++++++++++++-------------------
 drivers/misc/fastrpc.c     | 16 +++-----
 include/linux/dma-buf.h    |  1 +
 4 files changed, 67 insertions(+), 50 deletions(-)

-- 
2.34.1