Re: [Xen-devel] [PATCH] libxl: make sure buffer is null-terminated in libxl_read_file_contents

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Jun 27, 2018 at 07:08:02PM +0800, Robin Lee wrote:
> On Wed, Jun 27, 2018 at 6:58 PM, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> > On Wed, Jun 27, 2018 at 09:13:11AM +0000, Robin Lee wrote:
> >> On XenServer 7.1.1, we start a vm with XAPI but attach a block device with 
> >> xl.
> >> We create an empty json config for the vm with the content "{}\n" and then
> >> run 'xl block-attach':
> >>
> >>   #  xl block-attach 1 phy:/dev/loop0 xvdz w
> >>   libxl: error: libxl_json.c:950:libxl__json_parse: yajl error: parse 
> >> error: trailing garbage
> >>                                       {} K]
> >>                        (right here) ------^
> >>
> >>   libxl: error: libxl_json.c:1053:libxl__object_from_json: unable to 
> >> generate libxl__json_object from JSON representation of 
> >> libxl_domain_config.
> >>   libxl: error: libxl.c:1995:device_addrm_aocomplete: unable to add device
> >>   libxl_device_disk_add failed.
> >>
> >> After investigation, we found the buffer returned from 
> >> libxl_read_file_contents
> >> is not null-terminated. But later in libxl__object_from_json, the buffer 
> >> is expected to
> >> be null-terminated. So parsing may exceeded the end of file and get in to 
> >> uninisialized
> >> momery area.
> >>
> >> Signed-off-by: Robin Lee <robinlee.sysu@xxxxxxxxx>
> >
> > I can't seem to be able to reproduce this in upstream xen. Which version
> > of Xen does XenServer 7.1.1 have? You can get that from the output of
> > `xl info` -- look for xen_{major, minor, extra}.
> I also met a strange case. We didn't see this problem with Xen 4.7.1
> that released with
> XenServer 7.1.1. But since a recently hotfix from XenServer that upgraded Xen 
> to
> 4.7.4, this problem then shows up.
> 
> The version of yajl (yajl-2.0.4-4.el7.x86_64)  never changed.

As far as I can tell, the stored json file already contains trailing 0,
even in 4.7.4. There is nothing interesting between 4.7.1 and 4.7.4 in
that area of code.

I'm afraid I can't take in your patch before we understand why the code
doesn't function as expected. The problem is likely to be somewhere
else.

You can inspect the files under /var/lib/xen (or the location configured
by XenServer). The names are like userdata-d.X.$UUID.libxl-json.

One theory is the upgrade left some stale libraries which your xl then
used. Use ldd `which xl` to see which libraries it used. And make sure
it uses the latest ones that come with the upgrade.

Wei.

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