From 8c79a5b8fafdabc73eb1b3336564be37e8380190 Mon Sep 17 00:00:00 2001 Message-Id: <8c79a5b8fafdabc73eb1b3336564be37e8380190.1666356703.git.jbenc@redhat.com> From: Jiri Benc Date: Fri, 21 Oct 2022 11:21:11 +0200 Subject: [PATCH net] net: gso: fix panic on frag_list with mixed head alloc types Since commit 3dcbdb134f32 ("net: gso: Fix skb_segment splat when splitting gso_size mangled skb having linear-headed frag_list"), it is allowed to change gso_size of a GRO packet. However, that commit assumes that "checking the first list_skb member suffices; i.e if either of the list_skb members have non head_frag head, then the first one has too". It turns out this assumption does not hold. We've seen BUG_ON being hit in skb_segment when skbs on the frag_list had differing head_frag. That particular case was with vmxnet3; looking at the driver, it indeed uses different skb allocation strategies based on the packet size. The last packet in frag_list can thus be kmalloced if it is sufficiently small. And there's nothing preventing drivers from mixing things even more freely. There are three different locations where this can be fixed: (1) We could check head_frag in GRO and not allow GROing skbs with different head_frag. However, that would lead to performance regression (at least on vmxnet3) on normal forward paths with unmodified gso_size, where mixed head_frag is not a problem. (2) Set a flag in bpf_skb_net_grow and bpf_skb_net_shrink indicating that NETIF_F_SG is undesirable. That would need to eat a bit in sk_buff. Furthermore, that flag can be unset when all skbs on the frag_list are page backed. To retain good performance, bpf_skb_net_grow/shrink would have to walk the frag_list. (3) Walk the frag_list in skb_segment when determining whether NETIF_F_SG should be cleared. This of course slows things down. This patch implements (3). To limit the performance impact in skb_segment, the list is walked only for skbs with SKB_GSO_DODGY set that have gso_size changed. Normal paths thus will not hit it. Fixes: 3dcbdb134f32 ("net: gso: Fix skb_segment splat when splitting gso_size mangled skb having linear-headed frag_list") Signed-off-by: Jiri Benc --- net/core/skbuff.c | 36 +++++++++++++++++++----------------- 1 file changed, 19 insertions(+), 17 deletions(-) diff --git a/net/core/skbuff.c b/net/core/skbuff.c index 1d9719e72f9d..bbf3acff44c6 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -4134,23 +4134,25 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, int i = 0; int pos; - if (list_skb && !list_skb->head_frag && skb_headlen(list_skb) && - (skb_shinfo(head_skb)->gso_type & SKB_GSO_DODGY)) { - /* gso_size is untrusted, and we have a frag_list with a linear - * non head_frag head. - * - * (we assume checking the first list_skb member suffices; - * i.e if either of the list_skb members have non head_frag - * head, then the first one has too). - * - * If head_skb's headlen does not fit requested gso_size, it - * means that the frag_list members do NOT terminate on exact - * gso_size boundaries. Hence we cannot perform skb_frag_t page - * sharing. Therefore we must fallback to copying the frag_list - * skbs; we do so by disabling SG. - */ - if (mss != GSO_BY_FRAGS && mss != skb_headlen(head_skb)) - features &= ~NETIF_F_SG; + if ((skb_shinfo(head_skb)->gso_type & SKB_GSO_DODGY) && + mss != GSO_BY_FRAGS && mss != skb_headlen(head_skb)) { + struct sk_buff *check_skb; + + for (check_skb = list_skb; check_skb; check_skb = check_skb->next) { + if (skb_headlen(check_skb) && !check_skb->head_frag) { + /* gso_size is untrusted, and we have a frag_list with + * a linear non head_frag item. + * + * If head_skb's headlen does not fit requested gso_size, + * it means that the frag_list members do NOT terminate + * on exact gso_size boundaries. Hence we cannot perform + * skb_frag_t page sharing. Therefore we must fallback to + * copying the frag_list skbs; we do so by disabling SG. + */ + features &= ~NETIF_F_SG; + break; + } + } } __skb_push(head_skb, doffset); -- 2.37.3