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 2d76319c4c ("dma-buf: fix UAF in dma_buf_put()
tracepoint").

This undoes the FD_ADD() conversion done in commit 34dfce523c
("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: 281a226314 ("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

drivers/dma-buf/dma-buf.c

@@ -792,9 +792,13 @@ int dma_buf_fd(struct dma_buf *dmabuf, int flags)
 	if (!dmabuf || !dmabuf->file)
 		return -EINVAL;
 
-	fd = FD_ADD(flags, dmabuf->file);
+	fd = get_unused_fd_flags(flags);
+	if (fd < 0)
+		return fd;
+
 	DMA_BUF_TRACE(trace_dma_buf_fd, dmabuf, fd);
 
+	fd_install(fd, dmabuf->file);
 	return fd;
 }
 EXPORT_SYMBOL_NS_GPL(dma_buf_fd, "DMA_BUF");