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Date: Tue, 13 Dec 2022 23:32:09 -0700 From: "Subash Abhinov Kasiviswanathan (KS)" <quic_subashab@...cinc.com> To: Daniel Borkmann <daniel@...earbox.net>, <ast@...nel.org>, <andrii@...nel.org>, <martin.lau@...ux.dev>, <john.fastabend@...il.com>, <song@...nel.org>, <yhs@...com>, <kpsingh@...nel.org>, <sdf@...gle.com>, <haoluo@...gle.com>, <jolsa@...nel.org>, <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>, <pabeni@...hat.com>, <bpf@...r.kernel.org>, <netdev@...r.kernel.org> CC: Sean Tranchetti <quic_stranche@...cinc.com> Subject: Re: [PATCH net] filter: Account for tail adjustment during pull operations On 12/13/2022 3:42 PM, Daniel Borkmann wrote: > On 12/13/22 5:39 AM, Subash Abhinov Kasiviswanathan wrote: >> Extending the tail can have some unexpected side effects if a program is >> reading the content beyond the head skb headlen and all the skbs in the >> gso frag_list are linear with no head_frag - >> >> diff --git a/net/core/filter.c b/net/core/filter.c >> index bb0136e..d5f7f79 100644 >> --- a/net/core/filter.c >> +++ b/net/core/filter.c >> @@ -1654,6 +1654,20 @@ static DEFINE_PER_CPU(struct bpf_scratchpad, >> bpf_sp); >> static inline int __bpf_try_make_writable(struct sk_buff *skb, >> unsigned int write_len) >> { >> + struct sk_buff *list_skb = skb_shinfo(skb)->frag_list; >> + >> + if (skb_is_gso(skb) && list_skb && !list_skb->head_frag && >> + skb_headlen(list_skb)) { >> + int headlen = skb_headlen(skb); >> + int err = skb_ensure_writable(skb, write_len); >> + >> + /* pskb_pull_tail() has occurred */ >> + if (!err && headlen != skb_headlen(skb)) >> + skb_shinfo(skb)->gso_type |= SKB_GSO_DODGY; >> + >> + return err; >> + } > > __bpf_try_make_writable() does not look like the right location to me > given this is called also from various other places. bpf_skb_change_tail > has skb_gso_reset in there, potentially that or pskb_pull_tail itself > should mark it? Actually the program we used had BPF_FUNC_skb_pull_data and we put this check in __bpf_try_make_writable so that it would help out BPF_FUNC_skb_pull_data & other users of __bpf_try_make_writable. Having the check in __pskb_pull_tail seems preferable though. Could you tell if the following is acceptable as this works for us - diff --git a/net/core/skbuff.c b/net/core/skbuff.c index dfc14a7..0f60abb 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -2263,6 +2263,9 @@ void *__pskb_pull_tail(struct sk_buff *skb, int delta) insp = list; } else { /* Eaten partially. */ + if (skb_is_gso(skb) && !list->head_frag && + skb_headlen(list)) + skb_shinfo(skb)->gso_type |= SKB_GSO_DODGY; if (skb_shared(list)) { /* Sucks! We need to fork list. :-( */ > >> return skb_ensure_writable(skb, write_len); >> } >> >
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