xen-netback: don't produce zero-size SKB frags

stable inclusion
from stable-v4.19.306
commit 5bb8270789c88c0e4ad78c0de2f274f2275c7f6c
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I8YCSC
CVE: CVE-2023-46838

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5bb8270789c88c0e4ad78c0de2f274f2275c7f6c



--------------------------------

commit c7ec4f2d684e17d69bbdd7c4324db0ef5daac26a upstream.

While frontends may submit zero-size requests (wasting a precious slot),
core networking code as of at least 3ece7826 ("sock: skb_copy_ubufs
support for compound pages") can't deal with SKBs when they have all
zero-size fragments. Respond to empty requests right when populating
fragments; all further processing is fragment based and hence won't
encounter these empty requests anymore.

In a way this should have been that way from the beginning: When no data
is to be transferred for a particular request, there's not even a point
in validating the respective grant ref. That's no different from e.g.
passing NULL into memcpy() when at the same time the size is 0.

This is XSA-448 / CVE-2023-46838.

Cc: stable@vger.kernel.org
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>