linux-kernel - Re: [PATCH v4 1/4] device-dax: Don't leak kernel memory to user space after unloading kmem

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Date:   Fri, 8 May 2020 16:53:06 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org, kexec@...ts.infradead.org,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        stable@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v4 1/4] device-dax: Don't leak kernel memory to user
 space after unloading kmem

On Fri,  8 May 2020 10:42:14 +0200 David Hildenbrand <david@...hat.com> wrote:

> Assume we have kmem configured and loaded:
>   [root@...alhost ~]# cat /proc/iomem
>   ...
>   140000000-33fffffff : Persistent Memory$
>     140000000-1481fffff : namespace0.0
>     150000000-33fffffff : dax0.0
>       150000000-33fffffff : System RAM
> 
> Assume we try to unload kmem. This force-unloading will work, even if
> memory cannot get removed from the system.
>   [root@...alhost ~]# rmmod kmem
>   [   86.380228] removing memory fails, because memory [0x0000000150000000-0x0000000157ffffff] is onlined
>   ...
>   [   86.431225] kmem dax0.0: DAX region [mem 0x150000000-0x33fffffff] cannot be hotremoved until the next reboot
> 
> Now, we can reconfigure the namespace:
>   [root@...alhost ~]# ndctl create-namespace --force --reconfig=namespace0.0 --mode=devdax
>   [  131.409351] nd_pmem namespace0.0: could not reserve region [mem 0x140000000-0x33fffffff]dax
>   [  131.410147] nd_pmem: probe of namespace0.0 failed with error -16namespace0.0 --mode=devdax
>   ...
> 
> This fails as expected due to the busy memory resource, and the memory
> cannot be used. However, the dax0.0 device is removed, and along its name.
> 
> The name of the memory resource now points at freed memory (name of the
> device).
>   [root@...alhost ~]# cat /proc/iomem
>   ...
>   140000000-33fffffff : Persistent Memory
>     140000000-1481fffff : namespace0.0
>     150000000-33fffffff : �_�^7_��/_��wR��WQ���^��� ...
>     150000000-33fffffff : System RAM
> 
> We have to make sure to duplicate the string. While at it, remove the
> superfluous setting of the name and fixup a stale comment.
> 
> Fixes: 9f960da72b25 ("device-dax: "Hotremove" persistent memory that is used like normal RAM")
> Cc: stable@...r.kernel.org # v5.3

hm.

Is this really -stable material?  These are all privileged operations,
I expect?

Assuming "yes", I've queued this separately, staged for 5.7-rcX.  I'll
redo patches 2-4 as a three-patch series for 5.8-rc1.

> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Vishal Verma <vishal.l.verma@...el.com>
> Cc: Dave Jiang <dave.jiang@...el.com>
> Cc: Pavel Tatashin <pasha.tatashin@...een.com>

Reviewers, please ;)