netdev - Re: [PATCH net] net: hsr: avoid potential out-of-bound access in fill_frame

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Message-ID: <Z5DH4cIqndQOyUfX@PC-LX-SteWu>
Date: Wed, 22 Jan 2025 11:26:41 +0100
From: Stephan Wurm <stephan.wurm@...berle.de>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, eric.dumazet@...il.com,
	syzbot+671e2853f9851d039551@...kaller.appspotmail.com,
	WingMan Kwok <w-kwok2@...com>,
	Murali Karicheri <m-karicheri2@...com>,
	MD Danish Anwar <danishanwar@...com>, Jiri Pirko <jiri@...dia.com>,
	George McCollister <george.mccollister@...il.com>
Subject: Re: [PATCH net] net: hsr: avoid potential out-of-bound access in
 fill_frame_info()

Am 21. Jan 16:35 hat Eric Dumazet geschrieben:
> On Tue, Jan 21, 2025 at 4:15 PM Stephan Wurm <stephan.wurm@...berle.de> wrote:
>
> > I did some additional experiments.
> e
> > First, I was able to get v6.13 running on the system, although it did
> > not fix my issue.
> >
> > Then I played around with VLAN interfaces.
> >
> > I created an explicit VLAN interface on top of the prp interface. In that
> > case the VLAN header gets transparently attached to the tx frames and
> > forwarding through the interface layers works as expected.
> >
> > It was even possible to get my application working on top of the vlan
> > interface, but it resulted in two VLAN headers - the inner from the
> > application, the outer from the vlan interface.
> >
> > So when sending vlan tagged frames directly from an application through
> > a prp interface the mac_len field does not get updated, even though the
> > VLAN protocol header is properly detected; when sending frames through
> > an explicit vlan interface, the mac_len seems to be properly parsed
> > into the skb.
> >
> > Now I am running out of ideas how to proceed.
> >
> > For the time being I would locally revert this fix, to make my
> > application working again.
> > But I can support in testing proposed solutions.
>
>
> If mac_len can not be used, we need yet another pskb_may_pull()
>
> I am unsure why hsr_get_node() is working, since it also uses skb->mac_len
>
> Please test the following patch :
>
> diff --git a/net/hsr/hsr_forward.c b/net/hsr/hsr_forward.c
> index 87bb3a91598ee96b825f7aaff53aafb32ffe4f9..8942592130c151f2c948308e1ae16a6736822d5
> 100644
> --- a/net/hsr/hsr_forward.c
> +++ b/net/hsr/hsr_forward.c
> @@ -700,9 +700,11 @@ static int fill_frame_info(struct hsr_frame_info *frame,
>                 frame->is_vlan = true;
>
>         if (frame->is_vlan) {
> -               if (skb->mac_len < offsetofend(struct hsr_vlan_ethhdr, vlanhdr))
> +               if (pskb_may_pull(skb,
> +                                 skb_mac_offset(skb) +
> +                                 offsetofend(struct hsr_vlan_ethhdr, vlanhdr)))
>                         return -EINVAL;
> -               vlan_hdr = (struct hsr_vlan_ethhdr *)ethhdr;
> +               vlan_hdr = (struct hsr_vlan_ethhdr *)skb_mac_header(skb);
>                 proto = vlan_hdr->vlanhdr.h_vlan_encapsulated_proto;
>         }

Thank you very much! With this patch (slightly modified) everything works
as expected now :)

I only needed to invert the logic in the if clause, as otherwise all
proper VLAN frames were dropped from PRP.

Here is the modified version:

diff --git a/net/hsr/hsr_forward.c b/net/hsr/hsr_forward.c
index 87bb3a91598e..3491627bbaf4 100644
--- a/net/hsr/hsr_forward.c
+++ b/net/hsr/hsr_forward.c
@@ -700,9 +700,11 @@ static int fill_frame_info(struct hsr_frame_info *frame,
 		frame->is_vlan = true;

 	if (frame->is_vlan) {
-		if (skb->mac_len < offsetofend(struct hsr_vlan_ethhdr, vlanhdr))
+		if (!pskb_may_pull(skb,
+				   skb_mac_offset(skb) +
+				   offsetofend(struct hsr_vlan_ethhdr, vlanhdr)))
 			return -EINVAL;
-		vlan_hdr = (struct hsr_vlan_ethhdr *)ethhdr;
+		vlan_hdr = (struct hsr_vlan_ethhdr *)skb_mac_header(skb);
 		proto = vlan_hdr->vlanhdr.h_vlan_encapsulated_proto;
 	}