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Date:   Sat, 9 May 2020 07:40:33 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>, 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



> Am 09.05.2020 um 01:53 schrieb Andrew Morton <akpm@...ux-foundation.org>:
> 
> 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?

Yes, my thought was rather that an admin could bring the system into such a state (by mistake?). Let‘s see if somebody has a suggestion.

I guess if we were really unlucky, we could access invalid memory and trigger a BUG (e.g., page at the end of memory and does not contain a 0 byte).

> 
> 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.

Make sense, let‘s wait for review feedback, thanks!

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