Re: general protection fault in fuse_ctl_remove_conn

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

>From 9f41081f8bd6762a6f629e5e23e6d07a62bba69c Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Sat, 28 Apr 2018 11:24:09 +0900
Subject: [PATCH] fuse: don't keep inode-less dentry at fuse_ctl_add_dentry().

syzbot is reporting NULL pointer dereference at fuse_ctl_remove_conn() [1].
Since fc->ctl_ndents is incremented by fuse_ctl_add_conn() when new_inode()
failed, fuse_ctl_remove_conn() reaches an inode-less dentry and tries to
clear d_inode(dentry)->i_private field. Fix this by calling dput() rather
than incrementing fc->ctl_ndents when new_inode() failed.

[1] https://syzkaller.appspot.com/bug?id=f396d863067238959c91c0b7cfc10b163638cac6

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+32c236387d66c4516827@syzkaller.appspotmail.com>
Fixes: bafa96541b250a70 ("fuse: add control filesystem")
Cc: Miklos Szeredi <miklos@szeredi.hu>
---
 fs/fuse/control.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/control.c b/fs/fuse/control.c
index b9ea99c..a651f8e 100644
--- a/fs/fuse/control.c
+++ b/fs/fuse/control.c
@@ -211,10 +211,12 @@ static struct dentry *fuse_ctl_add_dentry(struct dentry *parent,
 	if (!dentry)
 		return NULL;
 
-	fc->ctl_dentry[fc->ctl_ndents++] = dentry;
 	inode = new_inode(fuse_control_sb);
-	if (!inode)
+	if (!inode) {
+		dput(dentry);
 		return NULL;
+	}
+	fc->ctl_dentry[fc->ctl_ndents++] = dentry;
 
 	inode->i_ino = get_next_ino();
 	inode->i_mode = mode;
-- 
1.8.3.1