Re: [PATCH bpf-next V3 1/6] bpf: Remove MTU check in __bpf_skb_max_len

Daniel Borkmann
2020-10-09T16:12:25Z

On 10/8/20 4:09 PM, Jesper Dangaard Brouer wrote:
> Multiple BPF-helpers that can manipulate/increase the size of the SKB uses
> __bpf_skb_max_len() as the max-length. This function limit size against
> the current net_device MTU (skb->dev->mtu).
>
> When a BPF-prog grow the packet size, then it should not be limited to the
> MTU. The MTU is a transmit limitation, and software receiving this packet
> should be allowed to increase the size. Further more, current MTU check in
> __bpf_skb_max_len uses the MTU from ingress/current net_device, which in
> case of redirects uses the wrong net_device.
>
> Keep a sanity max limit of IP6_MAX_MTU (under CONFIG_IPV6) which is 64KiB
> plus 40 bytes IPv6 header size. If compiled without IPv6 use IP_MAX_MTU.
>
> V3: replace __bpf_skb_max_len() with define and use IPv6 max MTU size.
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
> net/core/filter.c | 16 ++++++++--------
> 1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 05df73780dd3..ddc1f9ba89d1 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3474,11 +3474,11 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
> return 0;
> }
>
> -static u32 __bpf_skb_max_len(const struct sk_buff *skb)
> -{
> - return skb->dev ? skb->dev->mtu + skb->dev->hard_header_len :
> - SKB_MAX_ALLOC;
> -}
> +#ifdef IP6_MAX_MTU /* Depend on CONFIG_IPV6 */
> +#define BPF_SKB_MAX_LEN IP6_MAX_MTU
> +#else
> +#define BPF_SKB_MAX_LEN IP_MAX_MTU
> +#endif

Shouldn't that check on skb->protocol? The way I understand it is that a number of devices
including virtual ones use ETH_MAX_MTU as their dev->max_mtu, so the mtu must be in the range
of dev->min_mtu(=ETH_MIN_MTU), dev->max_mtu(=ETH_MAX_MTU). __dev_set_mtu() then sets the user
value to dev->mtu in the core if within this range. That means in your case skb->dev->hard_header_len
for example is left out, meaning if we go for some constant, that would need to be higher.

> BPF_CALL_4(sk_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
> u32, mode, u64, flags)
> @@ -3527,7 +3527,7 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
> {
> u32 len_cur, len_diff_abs = abs(len_diff);
> u32 len_min = bpf_skb_net_base_len(skb);
> - u32 len_max = __bpf_skb_max_len(skb);
> + u32 len_max = BPF_SKB_MAX_LEN;
> __be16 proto = skb->protocol;
> bool shrink = len_diff < 0;
> u32 off;
> @@ -3610,7 +3610,7 @@ static int bpf_skb_trim_rcsum(struct sk_buff *skb, unsigned int new_len)
> static inline int __bpf_skb_change_tail(struct sk_buff *skb, u32 new_len,
> u64 flags)
> {
> - u32 max_len = __bpf_skb_max_len(skb);
> + u32 max_len = BPF_SKB_MAX_LEN;
> u32 min_len = __bpf_skb_min_len(skb);
> int ret;
>
> @@ -3686,7 +3686,7 @@ static const struct bpf_func_proto sk_skb_change_tail_proto = {
> static inline int __bpf_skb_change_head(struct sk_buff *skb, u32 head_room,
> u64 flags)
> {
> - u32 max_len = __bpf_skb_max_len(skb);
> + u32 max_len = BPF_SKB_MAX_LEN;
> u32 new_len = skb->len + head_room;
> int ret;
>
>
>