bpf: Check for BPF_F_ADJ_ROOM_FIXED_GSO when bpf_skb_change_proto

In the forwarding path GRO -> BPF 6 to 4 -> GSO for TCP traffic, the
coalesced packet payload can be > MSS, but < MSS + 20.

bpf_skb_proto_6_to_4() will upgrade the MSS and it can be > the payload
length. After then tcp_gso_segment checks for the payload length if it
is <= MSS. The condition is causing the packet to be dropped.

tcp_gso_segment():
        [...]
        mss = skb_shinfo(skb)->gso_size;
        if (unlikely(skb->len <= mss))
                goto out;
        [...]

Allow to upgrade/downgrade MSS only when BPF_F_ADJ_ROOM_FIXED_GSO is
not set.

Signed-off-by: Dongseok Yi <dseok.yi@samsung.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/bpf/1620804453-57566-1-git-send-email-dseok.yi@samsung.com

diff --git a/net/core/filter.c b/net/core/filter.c
index cae56d08a67079bb2fa0335eec6afbd4c91c6ced..582ac196fd943ff9710dd9e15e7a54961224f9fa 100644 (file)
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3235,7 +3235,7 @@ static int bpf_skb_net_hdr_pop(struct sk_buff *skb, u32 off, u32 len)
        return ret;
 }
 
-static int bpf_skb_proto_4_to_6(struct sk_buff *skb)
+static int bpf_skb_proto_4_to_6(struct sk_buff *skb, u64 flags)
 {
        const u32 len_diff = sizeof(struct ipv6hdr) - sizeof(struct iphdr);
        u32 off = skb_mac_header_len(skb);
@@ -3264,7 +3264,9 @@ static int bpf_skb_proto_4_to_6(struct sk_buff *skb)
                }
 
                /* Due to IPv6 header, MSS needs to be downgraded. */
-               skb_decrease_gso_size(shinfo, len_diff);
+               if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
+                       skb_decrease_gso_size(shinfo, len_diff);
+
                /* Header must be checked, and gso_segs recomputed. */
                shinfo->gso_type |= SKB_GSO_DODGY;
                shinfo->gso_segs = 0;
@@ -3276,7 +3278,7 @@ static int bpf_skb_proto_4_to_6(struct sk_buff *skb)
        return 0;
 }
 
-static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
+static int bpf_skb_proto_6_to_4(struct sk_buff *skb, u64 flags)
 {
        const u32 len_diff = sizeof(struct ipv6hdr) - sizeof(struct iphdr);
        u32 off = skb_mac_header_len(skb);
@@ -3305,7 +3307,9 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
                }
 
                /* Due to IPv4 header, MSS can be upgraded. */
-               skb_increase_gso_size(shinfo, len_diff);
+               if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
+                       skb_increase_gso_size(shinfo, len_diff);
+
                /* Header must be checked, and gso_segs recomputed. */
                shinfo->gso_type |= SKB_GSO_DODGY;
                shinfo->gso_segs = 0;
@@ -3317,17 +3321,17 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
        return 0;
 }
 
-static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto)
+static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto, u64 flags)
 {
        __be16 from_proto = skb->protocol;
 
        if (from_proto == htons(ETH_P_IP) &&
              to_proto == htons(ETH_P_IPV6))
-               return bpf_skb_proto_4_to_6(skb);
+               return bpf_skb_proto_4_to_6(skb, flags);
 
        if (from_proto == htons(ETH_P_IPV6) &&
              to_proto == htons(ETH_P_IP))
-               return bpf_skb_proto_6_to_4(skb);
+               return bpf_skb_proto_6_to_4(skb, flags);
 
        return -ENOTSUPP;
 }
@@ -3337,7 +3341,7 @@ BPF_CALL_3(bpf_skb_change_proto, struct sk_buff *, skb, __be16, proto,
 {
        int ret;
 
-       if (unlikely(flags))
+       if (unlikely(flags & ~(BPF_F_ADJ_ROOM_FIXED_GSO)))
                return -EINVAL;
 
        /* General idea is that this helper does the basic groundwork
@@ -3357,7 +3361,7 @@ BPF_CALL_3(bpf_skb_change_proto, struct sk_buff *, skb, __be16, proto,
         * that. For offloads, we mark packet as dodgy, so that headers
         * need to be verified first.
         */
-       ret = bpf_skb_proto_xlat(skb, proto);
+       ret = bpf_skb_proto_xlat(skb, proto, flags);
        bpf_compute_data_pointers(skb);
        return ret;
 }