Re: [PATCH] proc: prevent mount proc on same mountpoint in one pid namespace

[Al Viro]

On Sat, Aug 21, 2021 at 01:31:05AM -0700, cgel.zte@xxxxxxxxx wrote:
> From: Yang Yang <yang.yang29@xxxxxxxxxx>
> 
> Patch "proc: allow to mount many instances of proc in one pid namespace"
> aims to mount many instances of proc on different mountpoint, see
> tools/testing/selftests/proc/proc-multiple-procfs.c.
> 
> But there is a side-effects, user can mount many instances of proc on
> the same mountpoint in one pid namespace, which is not allowed before.
> This duplicate mount makes no sense but wastes memory and CPU, and user
> may be confused why kernel allows it.
> 
> The logic of this patch is: when try to mount proc on /mnt, check if
> there is a proc instance mount on /mnt in the same pid namespace. If
> answer is yes, return -EBUSY.
> 
> Since this check can't be done in proc_get_tree(), which call
> get_tree_nodev() and will create new super_block unconditionally.
> And other nodev fs may faces the same case, so add a new hook in
> fs_context_operations.

NAK.  As attack prevention it's worthless (you can just bind-mount
a tmpfs directory between them).  Besides, filesystem does *not*
get to decide where it would be mounted.  Especially since it couldn't
rely upon that, anyway, what with mount --bind possible *after* it had
been initially mounted.