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

Jesper Dangaard Brouer
2020-10-08T14:09:15Z

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

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;