Re: [PATCH net] filter: Account for tail adjustment during pull operations
From: Daniel Borkmann <daniel@iogearbox.net>
Date: 2022-12-14 23:26:33
Also in:
bpf
On 12/14/22 7:32 AM, Subash Abhinov Kasiviswanathan (KS) wrote:
On 12/13/2022 3:42 PM, Daniel Borkmann wrote:quoted
On 12/13/22 5:39 AM, Subash Abhinov Kasiviswanathan wrote:quoted
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 -
Ah okay, that is good to know. The Fixes tag might have been misleading in that case. From what you describe it sounds like a generic __pskb_pull_tail() issue then? If so I'd go with the below for -net tree as a generic fix, yes.
quoted hunk ↗ jump to hunk
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. :-( */quoted
quoted
return skb_ensure_writable(skb, write_len); }