dma-buf: fix UAF in dma_buf_fd() tracepoint

dma-buf: fix UAF in dma_buf_fd() tracepoint

Once FD_ADD() returns, the fd is live in the file descriptor table
and a thread sharing that table can close() it before DMA_BUF_TRACE()
runs. The close drops the last reference, __fput() frees the dma_buf,
and the tracepoint then dereferences dmabuf to take dmabuf->name_lock
-- slab-use-after-free.

Split FD_ADD() back into get_unused_fd_flags() + fd_install() and
emit the tracepoint between them. While the fdtable slot is reserved
with a NULL file pointer, a racing close() returns -EBADF without
entering __fput(), so the dma_buf stays alive across the trace. Same
approach as commit 2d76319c4cbb ("dma-buf: fix UAF in dma_buf_put()
tracepoint").

This undoes the FD_ADD() conversion done in commit 34dfce523c90
("dma: convert dma_buf_fd() to FD_ADD()"); FD_ADD() has no place to
hook the tracepoint safely.

Reported-by: syzbot+7f4987d0afb97dd090cb@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7f4987d0afb97dd090cb
Fixes: 281a22631423 ("dma-buf: add some tracepoints to debug.")
Cc: stable@vger.kernel.org # 7.0.x
Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patch.msgid.link/20260523181446.69525-1-devnexen@gmail.com