xdp: Fix zero-size allocation warning in xskq_create()

Syzkaller reported the following issue:

  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 2807 at mm/vmalloc.c:3247 __vmalloc_node_range (mm/vmalloc.c:3361)
  Modules linked in:
  CPU: 0 PID: 2807 Comm: repro Not tainted 6.6.0-rc2+ #12
  Hardware name: Generic DT based system
  unwind_backtrace from show_stack (arch/arm/kernel/traps.c:258)
  show_stack from dump_stack_lvl (lib/dump_stack.c:107 (discriminator 1))
  dump_stack_lvl from __warn (kernel/panic.c:633 kernel/panic.c:680)
  __warn from warn_slowpath_fmt (./include/linux/context_tracking.h:153 kernel/panic.c:700)
  warn_slowpath_fmt from __vmalloc_node_range (mm/vmalloc.c:3361 (discriminator 3))
  __vmalloc_node_range from vmalloc_user (mm/vmalloc.c:3478)
  vmalloc_user from xskq_create (net/xdp/xsk_queue.c:40)
  xskq_create from xsk_setsockopt (net/xdp/xsk.c:953 net/xdp/xsk.c:1286)
  xsk_setsockopt from __sys_setsockopt (net/socket.c:2308)
  __sys_setsockopt from ret_fast_syscall (arch/arm/kernel/entry-common.S:68)

xskq_get_ring_size() uses struct_size() macro to safely calculate the
size of struct xsk_queue and q->nentries of desc members. But the
syzkaller repro was able to set q->nentries with the value initially
taken from copy_from_sockptr() high enough to return SIZE_MAX by
struct_size(). The next PAGE_ALIGN(size) is such case will overflow
the size_t value and set it to 0. This will trigger WARN_ON_ONCE in
vmalloc_user() -> __vmalloc_node_range().

The issue is reproducible on 32-bit arm kernel.

Fixes: 9f78bf33 ("xsk: support use vaddr as ring")
Reported-by:  <syzbot+fae676d3cf469331fc89@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/000000000000c84b4705fb31741e@google.com/T/


Reported-by:  <syzbot+b132693e925cbbd89e26@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/000000000000e20df20606ebab4f@google.com/T/


Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by:  <syzbot+fae676d3cf469331fc89@syzkaller.appspotmail.com>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://syzkaller.appspot.com/bug?extid=fae676d3cf469331fc89
Link: https://lore.kernel.org/bpf/20231007075148.1759-1-andrew.kanner@gmail.com