[PATCH v6 2/4] misc: fastrpc: don't publish fd before copy_to_user() succeeds

fastrpc_ioctl_alloc_dmabuf() calls dma_buf_fd() which installs the fd
into the caller's fd table before copy_to_user() copies the fd number
back to userspace.  If copy_to_user() fails, the fd is already visible
to other threads in the same process but the ioctl returns -EFAULT.
The existing comment in the code even acknowledges the problem:

  "The usercopy failed, but we can't do much about it, as dma_buf_fd()
   already called fd_install()..."

Now that dma_buf_fd_install() is available (introduced to fix the same
issue in dma-heap), apply the same pattern here: reserve the fd with
get_unused_fd_flags(), attempt copy_to_user(), and only on success call
dma_buf_fd_install() to publish it atomically with the tracepoint.  On
copy_to_user() failure, put_unused_fd() and dma_buf_put() cleanly
unwind without any user-visible side effects.

Fixes: 6cffd79504ce ("misc: fastrpc: Add support for dmabuf exporter")
Cc: [email protected]
Acked-by: Christian König <[email protected]>
Signed-off-by: Baineng Shou <[email protected]>
---
 drivers/misc/fastrpc.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index f3a49384586d..c5143cd25767 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -1709,24 +1709,20 @@ static int fastrpc_dmabuf_alloc(struct fastrpc_user *fl, char __user *argp)
 		return err;
 	}
 
-	bp.fd = dma_buf_fd(buf->dmabuf, O_ACCMODE);
+	bp.fd = get_unused_fd_flags(O_ACCMODE);
 	if (bp.fd < 0) {
 		dma_buf_put(buf->dmabuf);
-		return -EINVAL;
+		return bp.fd;
 	}
 
 	if (copy_to_user(argp, &bp, sizeof(bp))) {
-		/*
-		 * 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-		 * might already be closed and dmabuf no longer valid when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.
-		 */
+		put_unused_fd(bp.fd);
+		dma_buf_put(buf->dmabuf);
 		return -EFAULT;
 	}
 
+	dma_buf_fd_install(buf->dmabuf, bp.fd);
+
 	return 0;
 }
 
-- 
2.34.1