netdev - Re: [PATCH bpf-next 07/13] net: Clear skb metadata on handover from device to protocol

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87o6u5nfa5.fsf@cloudflare.com>
Date: Mon, 30 Jun 2025 22:30:26 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: bpf@...r.kernel.org,  Alexei Starovoitov <ast@...nel.org>,  Arthur Fabre
 <arthur@...hurfabre.com>,  Eric Dumazet <edumazet@...gle.com>,  Jakub
 Kicinski <kuba@...nel.org>,  Jesper Dangaard Brouer <hawk@...nel.org>,
  Jesse Brandeburg <jbrandeburg@...udflare.com>,  Joanne Koong
 <joannelkoong@...il.com>,  Lorenzo Bianconi <lorenzo@...nel.org>,  Toke
 Høiland-Jørgensen <thoiland@...hat.com>,  Yan Zhai
 <yan@...udflare.com>,
  netdev@...r.kernel.org,  kernel-team@...udflare.com,  Stanislav Fomichev
 <sdf@...ichev.me>
Subject: Re: [PATCH bpf-next 07/13] net: Clear skb metadata on handover from
 device to protocol

On Mon, Jun 30, 2025 at 09:25 AM -07, Stanislav Fomichev wrote:
> On 06/30, Jakub Sitnicki wrote:
>> With the extension of bpf_dynptr_from_skb(BPF_DYNPTR_F_SKB_METADATA), all
>> BPF programs authorized to call this kfunc now have access to the skb
>> metadata area.
>> 
>> These programs can read up to skb_shinfo(skb)->meta_len bytes located just
>> before skb_mac_header(skb), regardless of what data is currently there.
>> 
>> However, as the network stack processes the skb, headers may be added or
>> removed. Hence, we cannot assume that skb_mac_header() always marks the end
>> of the metadata area.
>> 
>> To avoid potential pitfalls, reset the skb metadata length to zero before
>> passing the skb to the protocol layers. This is a temporary measure until
>> we can make metadata persist through protocol processing.
>> 
>> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
>> ---
>>  net/core/dev.c | 1 +
>>  1 file changed, 1 insertion(+)
>> 
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index be97c440ecd5..4a2389997535 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -5839,6 +5839,7 @@ static int __netif_receive_skb_core(struct sk_buff **pskb, bool pfmemalloc,
>>  	}
>>  #endif
>>  	skb_reset_redirect(skb);
>> +	skb_metadata_clear(skb);
>
> And the assumption that it's not gonna break the existing cases is
> because there is currently no way to read that metadata out afterwards?

Correct, only tc_cls_act_is_valid_access tags the register as
PTR_TO_PACKET_META when loading __skb->data_meta, which allows access.

Something worth adding to the description. Thanks.