Re: [PATCH net] net: openvswitch: fix middle attribute validation in push_nsh() action

[Aaron Conole <aconole@redhat.com>]

Ilya Maximets <i.maximets@ovn.org> writes:

> The push_nsh() action structure looks like this:
>
>  OVS_ACTION_ATTR_PUSH_NSH(OVS_KEY_ATTR_NSH(OVS_NSH_KEY_ATTR_BASE,...))
>
> The outermost OVS_ACTION_ATTR_PUSH_NSH attribute is OK'ed by the
> nla_for_each_nested() inside __ovs_nla_copy_actions().  The innermost
> OVS_NSH_KEY_ATTR_BASE/MD1/MD2 are OK'ed by the nla_for_each_nested()
> inside nsh_key_put_from_nlattr().  But nothing checks if the attribute
> in the middle is OK.  We don't even check that this attribute is the
> OVS_KEY_ATTR_NSH.  We just do a double unwrap with a pair of nla_data()
> calls - first time directly while calling validate_push_nsh() and the
> second time as part of the nla_for_each_nested() macro, which isn't
> safe, potentially causing invalid memory access if the size of this
> attribute is incorrect.  The failure may not be noticed during
> validation due to larger netlink buffer, but cause trouble later during
> action execution where the buffer is allocated exactly to the size:
>
>  BUG: KASAN: slab-out-of-bounds in nsh_hdr_from_nlattr+0x1dd/0x6a0 [openvswitch]
>  Read of size 184 at addr ffff88816459a634 by task a.out/22624
>
>  CPU: 8 UID: 0 PID: 22624 6.18.0-rc7+ #115 PREEMPT(voluntary)
>  Call Trace:
>   <TASK>
>   dump_stack_lvl+0x51/0x70
>   print_address_description.constprop.0+0x2c/0x390
>   kasan_report+0xdd/0x110
>   kasan_check_range+0x35/0x1b0
>   __asan_memcpy+0x20/0x60
>   nsh_hdr_from_nlattr+0x1dd/0x6a0 [openvswitch]
>   push_nsh+0x82/0x120 [openvswitch]
>   do_execute_actions+0x1405/0x2840 [openvswitch]
>   ovs_execute_actions+0xd5/0x3b0 [openvswitch]
>   ovs_packet_cmd_execute+0x949/0xdb0 [openvswitch]
>   genl_family_rcv_msg_doit+0x1d6/0x2b0
>   genl_family_rcv_msg+0x336/0x580
>   genl_rcv_msg+0x9f/0x130
>   netlink_rcv_skb+0x11f/0x370
>   genl_rcv+0x24/0x40
>   netlink_unicast+0x73e/0xaa0
>   netlink_sendmsg+0x744/0xbf0
>   __sys_sendto+0x3d6/0x450
>   do_syscall_64+0x79/0x2c0
>   entry_SYSCALL_64_after_hwframe+0x76/0x7e
>   </TASK>
>
> Let's add some checks that the attribute is properly sized and it's
> the only one attribute inside the action.  Technically, there is no
> real reason for OVS_KEY_ATTR_NSH to be there, as we know that we're
> pushing an NSH header already, it just creates extra nesting, but
> that's how uAPI works today.  So, keeping as it is.
>
> Fixes: b2d0f5d5dc53 ("openvswitch: enable NSH support")
> Reported-by: Junvy Yang <zhuque@tencent.com>
> Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
> ---

Checked it out using also the userspace kmod test suite, and the
kernel selftest scripts.  Change makes sense to me.

Reviewed-by: Aaron Conole <aconole@redhat.com>