On Wed, Mar 21, 2018 at 1:36 PM, Yonghong Song <y...@fb.com> wrote:
> One of our in-house projects, bpf-based NAT, hits a kernel BUG_ON at
> function skb_segment(), line 3667. The bpf program attaches to
> clsact ingress, calls bpf_skb_change_proto to change protocol
> from ipv4 to ipv6 or from ipv6 to ipv4, and then calls bpf_redirect
> to send the changed packet out.
>
> 3472 struct sk_buff *skb_segment(struct sk_buff *head_skb,
> 3473 netdev_features_t features)
> 3474 {
> 3475 struct sk_buff *segs = NULL;
> 3476 struct sk_buff *tail = NULL;
> ...
> 3665 while (pos < offset + len) {
> 3666 if (i >= nfrags) {
> 3667 BUG_ON(skb_headlen(list_skb));
> 3668
> 3669 i = 0;
> 3670 nfrags = skb_shinfo(list_skb)->nr_frags;
> 3671 frag = skb_shinfo(list_skb)->frags;
> 3672 frag_skb = list_skb;
> ...
>
> call stack:
> ...
> #1 [ffff883ffef03558] __crash_kexec at ffffffff8110c525
> #2 [ffff883ffef03620] crash_kexec at ffffffff8110d5cc
> #3 [ffff883ffef03640] oops_end at ffffffff8101d7e7
> #4 [ffff883ffef03668] die at ffffffff8101deb2
> #5 [ffff883ffef03698] do_trap at ffffffff8101a700
> #6 [ffff883ffef036e8] do_error_trap at ffffffff8101abfe
> #7 [ffff883ffef037a0] do_invalid_op at ffffffff8101acd0
> #8 [ffff883ffef037b0] invalid_op at ffffffff81a00bab
> [exception RIP: skb_segment+3044]
> RIP: ffffffff817e4dd4 RSP: ffff883ffef03860 RFLAGS: 00010216
> RAX: 0000000000002bf6 RBX: ffff883feb7aaa00 RCX: 0000000000000011
> RDX: ffff883fb87910c0 RSI: 0000000000000011 RDI: ffff883feb7ab500
> RBP: ffff883ffef03928 R8: 0000000000002ce2 R9: 00000000000027da
> R10: 000001ea00000000 R11: 0000000000002d82 R12: ffff883f90a1ee80
> R13: ffff883fb8791120 R14: ffff883feb7abc00 R15: 0000000000002ce2
> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
> #9 [ffff883ffef03930] tcp_gso_segment at ffffffff818713e7
> --- <IRQ stack> ---
> ...
>
> The triggering input skb has the following properties:
> list_skb = skb->frag_list;
> skb->nfrags != NULL && skb_headlen(list_skb) != 0
> and skb_segment() is not able to handle a frag_list skb
> if its headlen (list_skb->len - list_skb->data_len) is not 0.
>
> This patch addressed the issue by handling skb_headlen(list_skb) != 0
> case properly if list_skb->head_frag is true, which is expected in
> most cases. The head frag is processed before list_skb->frags
> are processed.
>
> Reported-by: Diptanu Gon Choudhury <dipt...@fb.com>
> Signed-off-by: Yonghong Song <y...@fb.com>
> ---
> net/core/skbuff.c | 26 ++++++++++++++++++++------
> 1 file changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 715c134..23b317a 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3460,6 +3460,19 @@ void *skb_pull_rcsum(struct sk_buff *skb, unsigned int
> len)
> }
> EXPORT_SYMBOL_GPL(skb_pull_rcsum);
>
> +static inline skb_frag_t skb_head_frag_to_page_desc(struct sk_buff *frag_skb)
> +{
> + skb_frag_t head_frag;
> + struct page *page;
> +
> + page = virt_to_head_page(frag_skb->head);
> + head_frag.page.p = page;
> + head_frag.page_offset = frag_skb->data -
> + (unsigned char *)page_address(page);
> + head_frag.size = skb_headlen(frag_skb);
> + return head_frag;
> +}
> +
> /**
> * skb_segment - Perform protocol segmentation on skb.
> * @head_skb: buffer to segment
> @@ -3664,15 +3677,16 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>
> while (pos < offset + len) {
> if (i >= nfrags) {
> - BUG_ON(skb_headlen(list_skb));
> -
> i = 0;
> nfrags = skb_shinfo(list_skb)->nr_frags;
> frag = skb_shinfo(list_skb)->frags;
> - frag_skb = list_skb;
You could probably leave this line in place. No point in moving it.
> -
> - BUG_ON(!nfrags);
> + if (skb_headlen(list_skb)) {
> + BUG_ON(!list_skb->head_frag);
>
> + /* to make room for head_frag. */
> + i--; frag--;
Normally these should be two separate lines one for "i--;" and one for
"frag--;".
> + }
You could probably place the BUG_ON(!nfrags) in an else statement here
to handle the case where we have a potentially empty skb which would
be a bug.
> + frag_skb = list_skb;
> if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
> skb_zerocopy_clone(nskb, frag_skb,
> GFP_ATOMIC))
> @@ -3689,7 +3703,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
> goto err;
> }
>
> - *nskb_frag = *frag;
> + *nskb_frag = (i < 0) ?
> skb_head_frag_to_page_desc(frag_skb) : *frag;
> __skb_frag_ref(nskb_frag);
> size = skb_frag_size(nskb_frag);
>
> --
> 2.9.5
>
