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authorJesper Dangaard Brouer <brouer@redhat.com>2021-02-09 16:38:09 +0300
committerDaniel Borkmann <daniel@iogearbox.net>2021-02-13 03:14:08 +0300
commit6306c1189e77a513bf02720450bb43bd4ba5d8ae (patch)
treec7fdb60d6d7ec0a2d2ba83c4ec3c698bb9dd7895 /net
parent7d4553b69fb335496c597c31590e982485ebe071 (diff)
downloadlinux-6306c1189e77a513bf02720450bb43bd4ba5d8ae.tar.xz
bpf: Remove MTU check in __bpf_skb_max_len
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@redhat.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: John Fastabend <john.fastabend@gmail.com> Link: https://lore.kernel.org/bpf/161287788936.790810.2937823995775097177.stgit@firesoul
Diffstat (limited to 'net')
-rw-r--r--net/core/filter.c12
1 files changed, 4 insertions, 8 deletions
diff --git a/net/core/filter.c b/net/core/filter.c
index 57aaed478362..27022168fd78 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3552,11 +3552,7 @@ 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;
-}
+#define BPF_SKB_MAX_LEN SKB_MAX_ALLOC
BPF_CALL_4(sk_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
u32, mode, u64, flags)
@@ -3605,7 +3601,7 @@ 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;
@@ -3688,7 +3684,7 @@ 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;
@@ -3764,7 +3760,7 @@ 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;