misc: fastrpc: avoid double fput() on failed usercopy

If the copy back to userland fails for the FASTRPC_IOCTL_ALLOC_DMA_BUFF
ioctl(), we shouldn't assume that 'buf->dmabuf' is still valid. In fact,
dma_buf_fd() called fd_install() before, i.e. "consumed" one reference,
leaving us with none.

Calling dma_buf_put() will therefore put a reference we no longer own,
leading to a valid file descritor table entry for an already released
'file' object which is a straight use-after-free.

Simply avoid calling dma_buf_put() and rely on the process exit code to
do the necessary cleanup, if needed, i.e. if the file descriptor is
still valid.

Fixes: 6cffd79504ce ("misc: fastrpc: Add support for dmabuf exporter")
Acked-by: Christian König <[e-mail]>
Signed-off-by: Mathias Krause <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/fastrpc.c

···
 	}
 
 	if (copy_to_user(argp, &bp, sizeof(bp))) {
-		dma_buf_put(buf->dmabuf);
+		/*
+		 * The usercopy failed, but we can't do much about it, as
+		 * dma_buf_fd() already called fd_install() and made the
+		 * file descriptor accessible for the current process. It
+		 * might already be closed and dmabuf no longer valid when
+		 * we reach this point. Therefore "leak" the fd and rely on
+		 * the process exit path to do any required cleanup.
+		 */
 		return -EFAULT;
 	}