Message-ID: <71ec6168-f3ca-4454-8bfb-a8ae43a09159@ovn.org>
Date: Thu, 4 Dec 2025 14:50:57 +0100
From: Ilya Maximets <i.maximets@....org>
To: Eelco Chaudron <echaudro@...hat.com>
Cc: i.maximets@....org, netdev@...r.kernel.org,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, linux-kernel@...r.kernel.org,
 dev@...nvswitch.org, Aaron Conole <aconole@...hat.com>,
 Willy Tarreau <w@....eu>, LePremierHomme <kwqcheii@...ton.me>,
 Junvy Yang <zhuque@...cent.com>
Subject: Re: [PATCH net] net: openvswitch: fix middle attribute validation in
 push_nsh() action

On 12/4/25 2:22 PM, Eelco Chaudron wrote:
> 
> 
> On 4 Dec 2025, at 12:36, Ilya Maximets wrote:
> 
>> On 12/4/25 12:03 PM, Eelco Chaudron wrote:
>>>
>>>
>>> On 4 Dec 2025, at 11:53, Ilya Maximets wrote:
>>>
>>>> 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@...cent.com>
>>>> Signed-off-by: Ilya Maximets <i.maximets@....org>
>>>
>>> Thanks, Ilya, for fixing this. One small nit about logging, but overall it looks good to me.
>>>
>>> Acked-by: Eelco Chaudron echaudro@...hat.com
>>>
>>>> ---
>>>>  net/openvswitch/flow_netlink.c | 13 ++++++++++---
>>>>  1 file changed, 10 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/net/openvswitch/flow_netlink.c b/net/openvswitch/flow_netlink.c
>>>> index 1cb4f97335d8..2d536901309e 100644
>>>> --- a/net/openvswitch/flow_netlink.c
>>>> +++ b/net/openvswitch/flow_netlink.c
>>>> @@ -2802,13 +2802,20 @@ static int validate_and_copy_set_tun(const struct nlattr *attr,
>>>>  	return err;
>>>>  }
>>>>
>>>> -static bool validate_push_nsh(const struct nlattr *attr, bool log)
>>>> +static bool validate_push_nsh(const struct nlattr *a, bool log)
>>>>  {
>>>> +	struct nlattr *nsh_key = nla_data(a);
>>>>  	struct sw_flow_match match;
>>>>  	struct sw_flow_key key;
>>>>
>>>> +	/* There must be one and only one NSH header. */
>>>> +	if (!nla_ok(nsh_key, nla_len(a)) ||
>>>> +	    nla_total_size(nla_len(nsh_key)) != nla_len(a) ||
>>>> +	    nla_type(nsh_key) != OVS_KEY_ATTR_NSH)
>>>
>>> Should we consider adding some logging based on the log flag here? Not a blocker,
>>> just noticed that nsh_key_put_from_nlattr() logs similar validation cases and
>>> wondered if we want the same consistency.
>>
>> Our logging is not really consistent, we do not log in the same case for the
>> validate_set(), for example.  And I'm not sure if the log here would be useful
>> as it is very unlikely we can hit this condition without manually crafting the
>> attribute to be wrong.  We'll have a log later about garbage trailing data,
>> which should prompt a user to look at what they are sending down.
>>
>> In general, we should convert all the logging here into extack, as logs are
>> very inconvenient and not specific enough in most cases.
>>
>> But I can add something like this, if needed:
>>
>>   OVS_NLERR(log, "push_nsh: Expected a single NSH header");
>>
>> What do you think?
> 
> Reading your feedback, I’m fine with leaving it as is.

OK.  I will not send a v2 for this then, unless there will be some other feedback.

Best regards, Ilya Maximets.

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