Re: [Qemu-devel] [PATCH] vhost: secure vhost shared log files using argv paremeter

[Rafael David Tinoco]

Hello Michael, André,

Could you do a quick review before a final submission ?

http://paste.ubuntu.com/23446279/

- I split the commits into 1) bugfix, 2) new util with test, 3) vhostlog

The unit test is testing passing fds between 2 processes and asserting
contents of mmap buffer coming from the "vhostlog" util (mmap-file).

Your final comment on the "vhostlog" was:

>> Argv examples:
>>
>>     -netdev tap,id=net0,vhost=on
>>     -netdev tap,id=net0,vhost=on,vhostlog=/tmp/guest.log
>>     -netdev tap,id=net0,vhost=on,vhostlog=/tmp

(André) > Could it be only a filename? This would simplify testing.
(Michael) > When vhostlog is not specified, can we just use memfd as we did?

I'm going to change this to:

1 - if vhostlog is not provided shared log can't be used. Use memfd.

2 - for shared logs, vhostlog has to be provided as a "file" ?

Should i keep vhostlog being a directory also ? (i know we are unlinking the
file so might not be needed BUT a static file might have a race condition in
between different instances and providing a directory - that creates random
files on it - might be better approach).

Is there anything else ?

Thank you

Rafael Tinoco

On Mon, Oct 31, 2016 at 8:30 PM, Michael S. Tsirkin <address@hidden> wrote:
> On Mon, Oct 31, 2016 at 08:35:33AM -0200, Rafael David Tinoco wrote:
>> On Sun, Oct 30, 2016 at 5:26 PM, Michael S. Tsirkin <address@hidden> wrote:
>> >
>> > On Sat, Oct 22, 2016 at 07:00:41AM +0000, Rafael David Tinoco wrote:
>> > > Commit 31190ed7 added a migration blocker in vhost_dev_init() to
>> > > check if memfd would succeed. It is better if this blocker first
>> > > checks if vhost backend requires shared log. This will avoid a
>> > > situation where a blocker is added inappropriately (e.g. shared
>> > > log allocation fails when vhost backend doesn't support it).
>> >
>> > Sounds like a bugfix but I'm not sure. Can this part be split
>> > out in a patch by itself?
>>
>> Already sent some days ago (and pointed by Marc today).
>>
>> > > Commit: 35f9b6e added a fallback mechanism for systems not supporting
>> > > memfd_create syscall (started being supported since 3.17).
>> > >
>> > > Backporting memfd_create might not be accepted for distros relying
>> > > on older kernels. Nowadays there is no way for security driver
>> > > to discover memfd filename to be created: <tmpdir>/memfd-XXXXXX.
>> > >
>> > > Also, because vhost log file descriptors can be passed to other
>> > > processes, after discussion, we thought it is best to back mmap by
>> > > using files that can be placed into a specific directory: this commit
>> > > creates "vhostlog" argv parameter for such purpose. This will allow
>> > > security drivers to operate on those files appropriately.
>> > >
>> > > Argv examples:
>> > >
>> > >     -netdev tap,id=net0,vhost=on
>> > >     -netdev tap,id=net0,vhost=on,vhostlog=/tmp/guest.log
>> > >     -netdev tap,id=net0,vhost=on,vhostlog=/tmp
>> > >
>> > > For vhost backends supporting shared logs, if vhostlog is non-existent,
>> > > or a directory, random files are going to be created in the specified
>> > > directory (or, for non-existent, in tmpdir). If vhostlog is specified,
>> > > the filepath is always used when allocating vhost log files.
>> >
>> > When vhostlog is not specified, can we just use memfd as we did?
>> >
>>
>> This was my approach on a "pastebin" example before this patch (in the
>> discussion thread we had). Problem goes back to when vhost log file
>> descriptor is shared with some vhost-user implementation - like the
>> interface allows to - and the security driver labelling issue. IMO,
>> yes, we could let vhostlog to specify a log file, and, if not
>> specified, assume memfd is ok to be used.
>>
>> Please let me know if you - and Marc - want me to keep using memfd.
>> I'll create the mmap-file tests and files in a different commit, like
>> Marc has asked for, and will propose the patch again by the end of
>> this week.
>
> I think that the best approach is to allow passing in the fd,
> not the file path. If not passed, use memfd.
>
> --
> MST