netdev - Re: [PATCH net] bpf: Use mount_nodev not mount

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Date:	Fri, 20 May 2016 19:46:55 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ebiederm@...ssion.com
Cc:	hannes@...essinduktion.org, daniel@...earbox.net, ast@...nel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH net] bpf: Use mount_nodev not mount_ns to mount the bpf
 filesystem

From: ebiederm@...ssion.com (Eric W. Biederman)
Date: Fri, 20 May 2016 17:22:48 -0500

> 
> While reviewing the filesystems that set FS_USERNS_MOUNT I spotted the
> bpf filesystem.  Looking at the code I saw a broken usage of mount_ns
> with current->nsproxy->mnt_ns. As the code does not acquire a
> reference to the mount namespace it can not possibly be correct to
> store the mount namespace on the superblock as it does.
> 
> Replace mount_ns with mount_nodev so that each mount of the bpf
> filesystem returns a distinct instance, and the code is not buggy.
> 
> In discussion with Hannes Frederic Sowa it was reported that the use
> of mount_ns was an attempt to have one bpf instance per mount
> namespace, in an attempt to keep resources that pin resources from
> hiding.  That intent simply does not work, the vfs is not built to
> allow that kind of behavior.  Which means that the bpf filesystem
> really is buggy both semantically and in it's implemenation as it does
> not nor can it implement the original intent.
> 
> This change is userspace visible, but my experience with similar
> filesystems leads me to believe nothing will break with a model of each
> mount of the bpf filesystem is distinct from all others.
> 
> Fixes: b2197755b263 ("bpf: add support for persistent maps/progs")
> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Acked-by: Daniel Borkmann <daniel@...earbox.net>
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Applied and queued up for -stable, thanks everyone.