[PATCH] fuse: Clear setuid bit even in cache=never path

[Vivek Goyal <vgoyal@redhat.com>]

If fuse daemon is started with cache=never, fuse falls back to direct IO.
In that write path we don't call file_remove_privs() and that means setuid
bit is not cleared if unpriviliged user writes to a file with setuid bit set.

pjdfstest chmod test 12.t tests this and fails.

Fix this by calling fuse_remove_privs() even for direct I/O path.

I tested this as follows.

- Run fuse example pasthrough fs.

  $ passthrough_ll /mnt/pasthrough-mnt -o default_permissions,allow_other,cache=never
  $ mkdir /mnt/pasthrough-mnt/testdir
  $ cd /mnt/pasthrough-mnt/testdir
  $ prove -rv pjdfstests/tests/chmod/12.t

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 fs/fuse/file.c |   18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

Index: rhvgoyal-linux/fs/fuse/file.c
===================================================================
--- rhvgoyal-linux.orig/fs/fuse/file.c	2019-05-09 15:08:36.782054201 -0400
+++ rhvgoyal-linux/fs/fuse/file.c	2019-05-09 15:28:09.397054201 -0400
@@ -1456,14 +1456,18 @@ static ssize_t fuse_direct_write_iter(st
 	/* Don't allow parallel writes to the same file */
 	inode_lock(inode);
 	res = generic_write_checks(iocb, from);
-	if (res > 0) {
-		if (!is_sync_kiocb(iocb) && iocb->ki_flags & IOCB_DIRECT) {
-			res = fuse_direct_IO(iocb, from);
-		} else {
-			res = fuse_direct_io(&io, from, &iocb->ki_pos,
-					     FUSE_DIO_WRITE);
-		}
+	if (res <= 0)
+		goto out;
+
+	res = file_remove_privs(iocb->ki_filp);
+	if (res)
+		goto out;
+	if (!is_sync_kiocb(iocb) && iocb->ki_flags & IOCB_DIRECT) {
+		res = fuse_direct_IO(iocb, from);
+	} else {
+		res = fuse_direct_io(&io, from, &iocb->ki_pos, FUSE_DIO_WRITE);
 	}
+out:
 	fuse_invalidate_attr(inode);
 	if (res > 0)
 		fuse_write_update_size(inode, iocb->ki_pos);