From: Shmulik Ladkani shmulik@metanetworks.com
[ Upstream commit 3dcbdb134f329842a38f0e6797191b885ab00a00 ]
Historically, support for frag_list packets entering skb_segment() was limited to frag_list members terminating on exact same gso_size boundaries. This is verified with a BUG_ON since commit 89319d3801d1 ("net: Add frag_list support to skb_segment"), quote:
As such we require all frag_list members terminate on exact MSS boundaries. This is checked using BUG_ON. As there should only be one producer in the kernel of such packets, namely GRO, this requirement should not be difficult to maintain.
However, since commit 6578171a7ff0 ("bpf: add bpf_skb_change_proto helper"), the "exact MSS boundaries" assumption no longer holds: An eBPF program using bpf_skb_change_proto() DOES modify 'gso_size', but leaves the frag_list members as originally merged by GRO with the original 'gso_size'. Example of such programs are bpf-based NAT46 or NAT64.
This lead to a kernel BUG_ON for flows involving: - GRO generating a frag_list skb - bpf program performing bpf_skb_change_proto() or bpf_skb_adjust_room() - skb_segment() of the skb
See example BUG_ON reports in [0].
In commit 13acc94eff12 ("net: permit skb_segment on head_frag frag_list skb"), skb_segment() was modified to support the "gso_size mangling" case of a frag_list GRO'ed skb, but *only* for frag_list members having head_frag==true (having a page-fragment head).
Alas, GRO packets having frag_list members with a linear kmalloced head (head_frag==false) still hit the BUG_ON.
This commit adds support to skb_segment() for a 'head_skb' packet having a frag_list whose members are *non* head_frag, with gso_size mangled, by disabling SG and thus falling-back to copying the data from the given 'head_skb' into the generated segmented skbs - as suggested by Willem de Bruijn [1].
Since this approach involves the penalty of skb_copy_and_csum_bits() when building the segments, care was taken in order to enable this solution only when required: - untrusted gso_size, by testing SKB_GSO_DODGY is set (SKB_GSO_DODGY is set by any gso_size mangling functions in net/core/filter.c) - the frag_list is non empty, its item is a non head_frag, *and* the headlen of the given 'head_skb' does not match the gso_size.
[0] https://lore.kernel.org/netdev/20190826170724.25ff616f@pixies/ https://lore.kernel.org/netdev/9265b93f-253d-6b8c-f2b8-4b54eff1835c@fb.com/
[1] https://lore.kernel.org/netdev/CA+FuTSfVsgNDi7c=GUU8nMg2hWxF2SjCNLXetHeVPdnx...
Fixes: 6578171a7ff0 ("bpf: add bpf_skb_change_proto helper") Suggested-by: Willem de Bruijn willemdebruijn.kernel@gmail.com Cc: Daniel Borkmann daniel@iogearbox.net Cc: Eric Dumazet eric.dumazet@gmail.com Cc: Alexander Duyck alexander.duyck@gmail.com Signed-off-by: Shmulik Ladkani shmulik.ladkani@gmail.com Reviewed-by: Willem de Bruijn willemb@google.com Reviewed-by: Alexander Duyck alexander.h.duyck@linux.intel.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/core/skbuff.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+)
--- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -3514,6 +3514,25 @@ struct sk_buff *skb_segment(struct sk_bu int pos; int dummy;
+ 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; + } + __skb_push(head_skb, doffset); proto = skb_network_protocol(head_skb, &dummy); if (unlikely(!proto))