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Date:   Mon, 2 Apr 2018 21:37:46 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Eric Biggers <ebiggers3@gmail.com>
Cc:     syzbot <syzbot+f3bd89a5ab3266b10540@syzkaller.appspotmail.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, linux-nfs@vger.kernel.org
Subject: Re: BUG: corrupted list in __dentry_kill
Message-ID: <20180402203745.GE30522@ZenIV.linux.org.uk>
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On Mon, Apr 02, 2018 at 07:44:37AM +0100, Al Viro wrote:
> On Sun, Apr 01, 2018 at 02:48:54PM -0700, Eric Biggers wrote:
> > [+Cc linux-nfs]
> > > 
> > > [   42.965515] net/sunrpc/rpc_pipe.c: __rpc_create_common failed to allocate inode for dentry blocklayout
> > > [   42.967234] net/sunrpc/rpc_pipe.c: rpc_mkpipe_dentry() failed to create pipe nfs/blocklayout (errno = -12)
> 
> 	AFAICS, there's nothing to zero nn->bl_device_pipe->dentry after
> nfs4blocklayout_unregister_sb(), is there?  If nothing else, what's
> going to happen after mount/umount/mount with failing
> nfs4blocklayout_register_sb()?  AFAICS, we'll have stale pointer to
> dentry sitting in nn->bl_device_pipe->dentry, and call rpc_unlink()
> on it while cleaning up after the failing mount.
> 
> 	I don't think that's all there is to it, but it does smell like
> a bug.

That's not all.  Making nfs4blocklayout_register_sb() immediately fail
(without doing anything) leads to that oops on the very first attempt
to mount rpc_pipefs.

Matter of fact, rpc_gssd_dummy_depopulate() is garbage.  I don't know
how it had been tested, but it will do an extra dput() of gssd_dentry
whenever it's called.  _Any_ failure that sends us to err_depopulate:
(== any failure in rpc_pipefs_notifier_list callbacks) means an oops
there on attempt to dput() an already freed dentry.

Hell, turn that if (err) goto?err_depopulate; into
if (err || !strcmp(current->comm, "bugger")) goto err_depopulate;
cp /bin/mount /root/bugger, then boot with init=/bin/sh
and
cd /root; ./bugger -t rpc_pipefs none /mnt
will trigger just that.