Re: [PATCH] fuse: don't keep inode-less dentry at fuse_ctl_add_dentry().

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, May 09, 2018 at 07:58:58PM +0900, Tetsuo Handa wrote:
> >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.

That looks bloody awful.  Sure, everything that accesses fc->ctl_dentry is
synchronous with this, but it would've been much easier to follow if
shoving dentry into that array happened only after it's been fully set
up.

Incidentally, there's a nasty headache waiting to happen in that code -
consider a twit mounting something on that.  And think what happens when
connection gets shut down...