linux-kernel - Re: [PATCH net] net: openvswitch: fix middle attribute validation in push

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Message-ID: <9A785713-3692-43A7-BD08-652DC1248955@redhat.com>
Date: Thu, 04 Dec 2025 12:03:07 +0100
From: Eelco Chaudron <echaudro@...hat.com>
To: Ilya Maximets <i.maximets@....org>
Cc: 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 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.

> +		return false;
> +
>  	ovs_match_init(&match, &key, true, NULL);
> -	return !nsh_key_put_from_nlattr(attr, &match, false, true, log);
> +	return !nsh_key_put_from_nlattr(nsh_key, &match, false, true, log);
>  }
>
>  /* Return false if there are any non-masked bits set.
> @@ -3389,7 +3396,7 @@ static int __ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
>  					return -EINVAL;
>  			}
>  			mac_proto = MAC_PROTO_NONE;
> -			if (!validate_push_nsh(nla_data(a), log))
> +			if (!validate_push_nsh(a, log))
>  				return -EINVAL;
>  			break;
>
> -- 
> 2.51.1