Quoting Stefan Berger (stefanb@xxxxxxxxxxxxxxxxxx):
> On 07/11/2017 11:45 PM, Serge E. Hallyn wrote:
> >Quoting Stefan Berger (Stefan Bergerstefanb@xxxxxxxxxxxxxxxxxx):
> >>+/*
> >>+ * xattr_list_userns_rewrite - Rewrite list of xattr names for user namespaces
> >>+ * or determine needed size for attribute list
> >>+ * in case size == 0
> >>+ *
> >>+ * In a user namespace we do not present all extended attributes to the
> >>+ * user. We filter out those that are in the list of userns supported xattr.
> >>+ * Besides that we filter out those with @uid=<uid> when there is no mapping
> >>+ * for that uid in the current user namespace.
> >>+ *
> >>+ * @list: list of 0-byte separated xattr names
> >>+ * @size: the size of the list; may be 0 to determine needed list size
> >>+ * @list_maxlen: allocated buffer size of list
> >>+ */
> >>+static ssize_t
> >>+xattr_list_userns_rewrite(char *list, ssize_t size, size_t list_maxlen)
> >>+{
> >>+ char *nlist = NULL;
> >>+ size_t s_off, len, nlen;
> >>+ ssize_t d_off;
> >>+ char *name, *newname;
> >>+
> >>+ if (!list || size < 0 || current_user_ns() == &init_user_ns)
> >>+ return size;
> >>+
> >>+ if (size) {
> >>+ nlist = kmalloc(list_maxlen, GFP_KERNEL);
> >>+ if (!nlist)
> >>+ return -ENOMEM;
> >>+ }
> >>+
> >>+ s_off = d_off = 0;
> >>+ while (s_off < size || size == 0) {
> >>+ name = &list[s_off];
> >>+
> >>+ len = strlen(name);
> >>+ if (!len)
> >>+ break;
> >>+
> >>+ if (xattr_is_userns_supported(name, false) >= 0)
> >>+ newname = name;
> >>+ else {
> >>+ newname = xattr_rewrite_userns_xattr(name);
> >Why are you doing this here? If we get here it means that
> >xattr_is_userns_supported() returned < 0, meaning name is
> >not userns-supported. So xattr_rewrite_userns_xattr() will
> >just return name. Am I missing something?
>
> xattr_is_userns_support(name, false) does a _full string match_
> rather than a prefix match and will only return >= 0 for
> security.capability. This case handles the hosts's
> security.capability which 'shines through' for read and needs to be
> listed. Only in this case we set newname=name.
Ah, right.
I think it would be worth #defining XATTR_PREFIX_SEARCH and
XATTR_FULLNAME_SEARCH or something. Or maybe not, maybe I was
just being dense.
> In the else branch we handle security.capability@uid=1000 and
> rewrite that to security.capability for root mapping to uid=1000.
>
> >
> >>+ if (IS_ERR(newname)) {
> >>+ d_off = PTR_ERR(newname);
> >>+ goto out_free;
> >>+ }
> >>+ }
> >>+ if (newname && !xattr_list_contains(nlist, d_off, newname)) {
> >Now here, if name was recalculated to @newname, and @newname is
> >found in the nlist, that should raise an error right? Something
> >fishy is going on?
>
> If security.capability is set on a file but the container doesn't
> have security.capability@uid=1000, we still need to list the former
> here. However, we end up with duplicates if security.capability is
> there and security.capability@uid=1000 is also there and root is
> mapped to uid=1000. Both would be shown as security.capability
> inside the container. In this case we need to filter.
Gotcha, thanks.
> I think the code is correct. More problematic is a memory leak in
> the error case. Will fix that.
Great.
> >
> >>+ nlen = strlen(newname);
> >>+
> >>+ if (nlist) {
> >>+ if (nlen + 1 > list_maxlen)
> >d_off needs to be set to -ERANGE here.
>
> Fixed.
Great, thanks.
-serge
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