[PATCH AUTOSEL 5.4 17/74] bpf: Check for BPF_F_ADJ_ROOM_FIXED_GSO when bpf_skb_change_proto

Sasha Levin
2021-07-06T12:01:36Z

From: Dongseok Yi <dseok.yi@samsung.com>

[ Upstream commit fa7b83bf3b156c767f3e4a25bbf3817b08f3ff8e ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/core/filter.c | 22 +++++++++++++---------
1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 108bcf600052..823e77846525 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2833,7 +2833,7 @@ 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);
@@ -2862,7 +2862,9 @@ 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;
@@ -2874,7 +2876,7 @@ 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);
@@ -2903,7 +2905,9 @@ 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;
@@ -2915,17 +2919,17 @@ 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;
}
@@ -2935,7 +2939,7 @@ 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
@@ -2955,7 +2959,7 @@ 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;
}
--
2.30.2