Re: [PATCH 5/5] Allow all user to create a file under the directory /var/lib/xen

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 30/12/2015 05:25, Wen Congyang wrote:
> On 12/30/2015 12:11 PM, Doug Goldstein wrote:
>> On 12/29/15 8:39 PM, Wen Congyang wrote:
>>> We may use non-root user to run qemu, and the qemu needs to write
>>> save file to /var/lib/xen. So we should allow all user to create
>>> a file under the directory /var/lib/xen
>>>
>>> Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
>>> ---
>>>   tools/Makefile | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/tools/Makefile b/tools/Makefile
>>> index 820ca40..402b417 100644
>>> --- a/tools/Makefile
>>> +++ b/tools/Makefile
>>> @@ -60,7 +60,7 @@ build all: subdirs-all
>>>   install: subdirs-install
>>>   	$(INSTALL_DIR) -m 700 $(DESTDIR)$(XEN_DUMP_DIR)
>>>   	$(INSTALL_DIR) $(DESTDIR)/var/log/xen
>>> -	$(INSTALL_DIR) $(DESTDIR)/var/lib/xen
>>> +	$(INSTALL_DIR) -m 777 $(DESTDIR)/var/lib/xen
>>>   
>>>   .PHONY: uninstall
>>>   uninstall: D=$(DESTDIR)
>>>
>> I could be wrong but this doesn't seem like something that you'd want to
>> do given what's stored in there. Could you do something with permissions
>> on sub-directories to achieve what you need?
>>
> The save file's path is:
> #define LIBXL_DEVICE_MODEL_SAVE_FILE "/var/lib/xen/qemu-save" /* .$domid */
>
> So all user must have write permission on the directory /var/lib/xen/, otherwise,
> the migration will fail.

For now, I would avoid running qemu as a non-root user.  It doesn't gain 
you any meaninful security at present (at the expense of a warning which 
can't be turned off).

As to this bug, marking the directory 0777 is not an option, as save 
records necessarily contain sensitive data.

Longterm, (and already identified in one of the threads in the past), 
the best course of action is to switch away from having files, and 
passing file descriptors instead.  This is more flexible (currently 
libxl can't function on a read-only root filesystem), and would allow a 
privileged entity to open the file descriptor and pass it to a 
non-privileged entity to use.  This allows the non-privileged entity to 
function, and maintains security.

~Andrew