Re: [PATCH] namespace.c: fix mnt_namespace clearing

[Jamie Lokier <jamie@shareable.org>]

Miklos Szeredi wrote:
> This patch clears mnt_namespace on unmount.
> 
> Not clearing mnt_namespace has two effects:
> 
>    1) It is possible to attach a new mount to a detached mount,
>       because check_mnt() returns true.
> 
>       This means, that when no other references to the detached mount
>       remain, it still can't be freed.  This causes a resource leak,
>       and possibly un-removable modules.
> 
>    2) If mnt_namespace is dereferenced (only in mark_mounts_for_expiry())
>       after the namspace has been freed, it can cause an Oops, memory
>       corruption, etc.
> 
> 1) has been tested before and after the patch, 2) is only speculation.

You're right - I was just thinking the same thing.  There is also
another side effect, which is ironic in the context of recent discussion:

     3) Because mnt_namespace may refer to freed memory, it may refer
        to memory that's then allocated for _another_ namespace.  So the
        check for mounting in the correct namespace which prevents
	recursive bind mounts could erronously _allow_ the recursive
	bind to succeed (though without taking the correct lock).

-- Jamie