On 11/06/2018 09:14 AM, Paul Durrant wrote:
>> -----Original Message-----
>> From: Xen-devel [mailto:[email protected]] On Behalf
>> Of George Dunlap
>> Sent: 05 November 2018 18:07
>> To: [email protected]
>> Cc: Anthony Perard <[email protected]>; Ian Jackson
>> <[email protected]>; Wei Liu <[email protected]>; George Dunlap
>> <[email protected]>
>> Subject: [Xen-devel] [PATCH v4 3/6] tools/dm_restrict: Ask QEMU to chroot
>>
>> When dm_restrict is enabled, ask QEMU to chroot into an empty directory.
>>
>> * Create /var/run/qemu/root-domid (deleting the old one if it's there)
>
> This does not appear to match the code: the path should be
> /var/run/qemu-root-<domid> AFAICT
Indeed, I forgot to update this. I can fix this up on check-in.
>> diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
>> index 26eb16af34..ad3efcc783 100644
>> --- a/tools/libxl/libxl_dm.c
>> +++ b/tools/libxl/libxl_dm.c
>> @@ -1410,9 +1410,48 @@ static int
>> libxl__build_device_model_args_new(libxl__gc *gc,
>> }
>> }
>>
>> - if (libxl_defbool_val(b_info->dm_restrict))
>> + if (libxl_defbool_val(b_info->dm_restrict)) {
>> + char *chroot_dir = GCSPRINTF("%s/qemu-root-%d",
>> + libxl__run_dir_path(),
>> guest_domid);
>> + int r;
>> +
>> flexarray_append(dm_args, "-xen-domid-restrict");
>>
>> + /*
>> + * Run QEMU in a chroot at XEN_RUN_DIR/qemu-root-%d
>
> Maybe '<domid>' in the comment rather than '%d'?
Maybe. :-)
>> + *
>> + * There is no library function to do the equivalent of `rm
>> + * -rf`. However deprivileged QEMU in theory shouldn't be
>> + * able to write any files, as the chroot would be owned by
>> + * root, but it would be running as an unprivileged process.
>> + * So in theory, old chroots should always be empty.
>
> How does logging work if QEMU can't write to the chroot? I assume we are
> relying on stderr? Does using syslog still work?
Everything QEMU needs access to (including vnc sockets, qmp sockets, &c)
must either be opened before the chroot happens, or passed to QEMU as an
fd via qmp. In the case of logging, this happens through stderr; but if
you search for 'chroot' in the design document you'll get a couple of
examples of different issues that need to be addressed (including
inserting a cd-rom and dealing with migration).
-George
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