fuse: Fix oops at process_init_reply()

BugLink: https://bugs.launchpad.net/bugs/1792392

commit e8f3bd77 upstream.

syzbot is hitting NULL pointer dereference at process_init_reply().
This is because deactivate_locked_super() is called before response for
initial request is processed.

Fix this by aborting and waiting for all requests (including FUSE_INIT)
before resetting fc->sb.

Original patch by Tetsuo Handa <penguin-kernel@I-love.SKAURA.ne.jp>.
Reported-by: syzbot <syzbot+b62f08f4d5857755e3bc@syzkaller.appspotmail.com>
Fixes: e27c9d38 ("fuse: fuse: add time_gran to INIT_OUT")
Cc: <stable@vger.kernel.org> # v3.19
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/fuse/inode.c

@@ -384,11 +384,6 @@ static void fuse_put_super(struct super_block *sb)
 {
 	struct fuse_conn *fc = get_fuse_conn_super(sb);
 
-	fuse_send_destroy(fc);
-
-	fuse_abort_conn(fc);
-	fuse_wait_aborted(fc);
-
 	mutex_lock(&fuse_mutex);
 	list_del(&fc->entry);
 	fuse_ctl_remove_conn(fc);
@@ -1193,16 +1188,25 @@ static struct dentry *fuse_mount(struct file_system_type *fs_type,
 	return mount_nodev(fs_type, flags, raw_data, fuse_fill_super);
 }
 
-static void fuse_kill_sb_anon(struct super_block *sb)
+static void fuse_sb_destroy(struct super_block *sb)
 {
 	struct fuse_conn *fc = get_fuse_conn_super(sb);
 
 	if (fc) {
+		fuse_send_destroy(fc);
+
+		fuse_abort_conn(fc);
+		fuse_wait_aborted(fc);
+
 		down_write(&fc->killsb);
 		fc->sb = NULL;
 		up_write(&fc->killsb);
 	}
+}
 
+static void fuse_kill_sb_anon(struct super_block *sb)
+{
+	fuse_sb_destroy(sb);
 	kill_anon_super(sb);
 }
 
@@ -1225,14 +1229,7 @@ static struct dentry *fuse_mount_blk(struct file_system_type *fs_type,
 
 static void fuse_kill_sb_blk(struct super_block *sb)
 {
-	struct fuse_conn *fc = get_fuse_conn_super(sb);
-
-	if (fc) {
-		down_write(&fc->killsb);
-		fc->sb = NULL;
-		up_write(&fc->killsb);
-	}
-
+	fuse_sb_destroy(sb);
 	kill_block_super(sb);
 }