From: Jesper Dangaard Brouer <brouer@xxxxxxxxxx> commit 6306c1189e77a513bf02720450bb43bd4ba5d8ae upstream. 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. This patch keeps a sanity max limit of SKB_MAX_ALLOC (16KiB). The real limit is elsewhere in the system. Jesper's testing[1] showed it was not possible to exceed 8KiB when expanding the SKB size via BPF-helper. The limiting factor is the define KMALLOC_MAX_CACHE_SIZE which is 8192 for SLUB-allocator (CONFIG_SLUB) in-case PAGE_SIZE is 4096. This define is in-effect due to this being called from softirq context see code __gfp_pfmemalloc_flags() and __do_kmalloc_node(). Jakub's testing showed that frames above 16KiB can cause NICs to reset (but not crash). Keep this sanity limit at this level as memory layer can differ based on kernel config. [1] https://github.com/xdp-project/bpf-examples/tree/master/MTU-tests Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx> Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx> Acked-by: John Fastabend <john.fastabend@xxxxxxxxx> Link: https://lore.kernel.org/bpf/161287788936.790810.2937823995775097177.stgit@firesoul Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- net/core/filter.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) --- a/net/core/filter.c +++ b/net/core/filter.c @@ -2120,10 +2120,7 @@ static u32 __bpf_skb_min_len(const struc return min_len; } -static u32 __bpf_skb_max_len(const struct sk_buff *skb) -{ - return skb->dev->mtu + skb->dev->hard_header_len; -} +#define BPF_SKB_MAX_LEN SKB_MAX_ALLOC static int bpf_skb_grow_rcsum(struct sk_buff *skb, unsigned int new_len) { @@ -2144,7 +2141,7 @@ static int bpf_skb_trim_rcsum(struct sk_ BPF_CALL_3(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;
