[PATCH AUTOSEL 5.7 270/274] bpf: Fix up bpf_skb_adjust_room helper's skb csum setting

Sasha Levin
2020-06-09T00:44:33Z

From: Daniel Borkmann <daniel@iogearbox.net>

[ Upstream commit 836e66c218f355ec01ba57671c85abf32961dcea ]

Lorenz recently reported:

In our TC classifier cls_redirect [0], we use the following sequence of
helper calls to decapsulate a GUE (basically IP + UDP + custom header)
encapsulated packet:

bpf_skb_adjust_room(skb, -encap_len, BPF_ADJ_ROOM_MAC, BPF_F_ADJ_ROOM_FIXED_GSO)
bpf_redirect(skb->ifindex, BPF_F_INGRESS)

It seems like some checksums of the inner headers are not validated in
this case. For example, a TCP SYN packet with invalid TCP checksum is
still accepted by the network stack and elicits a SYN ACK. [...]

That is, we receive the following packet from the driver:

| ETH | IP | UDP | GUE | IP | TCP |
skb->ip_summed == CHECKSUM_UNNECESSARY

ip_summed is CHECKSUM_UNNECESSARY because our NICs do rx checksum offloading.
On this packet we run skb_adjust_room_mac(-encap_len), and get the following:

| ETH | IP | TCP |
skb->ip_summed == CHECKSUM_UNNECESSARY

Note that ip_summed is still CHECKSUM_UNNECESSARY. After bpf_redirect()'ing
into the ingress, we end up in tcp_v4_rcv(). There, skb_checksum_init() is
turned into a no-op due to CHECKSUM_UNNECESSARY.

The bpf_skb_adjust_room() helper is not aware of protocol specifics. Internally,
it handles the CHECKSUM_COMPLETE case via skb_postpull_rcsum(), but that does
not cover CHECKSUM_UNNECESSARY. In this case skb->csum_level of the original
skb prior to bpf_skb_adjust_room() call was 0, that is, covering UDP. Right now
there is no way to adjust the skb->csum_level. NICs that have checksum offload
disabled (CHECKSUM_NONE) or that support CHECKSUM_COMPLETE are not affected.

Use a safe default for CHECKSUM_UNNECESSARY by resetting to CHECKSUM_NONE and
add a flag to the helper called BPF_F_ADJ_ROOM_NO_CSUM_RESET that allows users
from opting out. Opting out is useful for the case where we don't remove/add
full protocol headers, or for the case where a user wants to adjust the csum
level manually e.g. through bpf_csum_level() helper that is added in subsequent
patch.

The bpf_skb_proto_{4_to_6,6_to_4}() for NAT64/46 translation from the BPF
bpf_skb_change_proto() helper uses bpf_skb_net_hdr_{push,pop}() pair internally
as well but doesn't change layers, only transitions between v4 to v6 and vice
versa, therefore no adoption is required there.

[0] https://lore.kernel.org/bpf/20200424185556.7358-1-lmb@cloudflare.com/

Fixes: 2be7e212d541 ("bpf: add bpf_skb_adjust_room helper")
Reported-by: Lorenz Bauer <lmb@cloudflare.com>
Reported-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/bpf/CACAyw9-uU_52esMd1JjuA80fRPHJv5vsSg8GnfW3t_qDU4aVKQ@mail.gmail.com/
Link: https://lore.kernel.org/bpf/11a90472e7cce83e76ddbfce81fdfce7bfc68808.1591108731.git.daniel@iogearbox.net
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/skbuff.h | 8 ++++++++
include/uapi/linux/bpf.h | 8 ++++++++
net/core/filter.c | 8 ++++++--
tools/include/uapi/linux/bpf.h | 8 ++++++++
4 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 3000c526f552..7e737a94bc63 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3945,6 +3945,14 @@ static inline void __skb_incr_checksum_unnecessary(struct sk_buff *skb)
}
}

+static inline void __skb_reset_checksum_unnecessary(struct sk_buff *skb)
+{
+ if (skb->ip_summed == CHECKSUM_UNNECESSARY) {
+ skb->ip_summed = CHECKSUM_NONE;
+ skb->csum_level = 0;
+ }
+}
+
/* Check if we need to perform checksum complete validation.
*
* Returns true if checksum complete is needed, false otherwise
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index f9b7fdd951e4..c01de7924e97 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1589,6 +1589,13 @@ union bpf_attr {
* Grow or shrink the room for data in the packet associated to
* *skb* by *len_diff*, and according to the selected *mode*.
*
+ * By default, the helper will reset any offloaded checksum
+ * indicator of the skb to CHECKSUM_NONE. This can be avoided
+ * by the following flag:
+ *
+ * * **BPF_F_ADJ_ROOM_NO_CSUM_RESET**: Do not reset offloaded
+ * checksum data of the skb to CHECKSUM_NONE.
+ *
* There are two supported modes at this time:
*
* * **BPF_ADJ_ROOM_MAC**: Adjust room at the mac layer
@@ -3235,6 +3242,7 @@ enum {
BPF_F_ADJ_ROOM_ENCAP_L3_IPV6 = (1ULL << 2),
BPF_F_ADJ_ROOM_ENCAP_L4_GRE = (1ULL << 3),
BPF_F_ADJ_ROOM_ENCAP_L4_UDP = (1ULL << 4),
+ BPF_F_ADJ_ROOM_NO_CSUM_RESET = (1ULL << 5),
};

enum {
diff --git a/net/core/filter.c b/net/core/filter.c
index 5cc9276f1023..11b97c31bca5 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3124,7 +3124,8 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
{
int ret;

- if (flags & ~BPF_F_ADJ_ROOM_FIXED_GSO)
+ if (unlikely(flags & ~(BPF_F_ADJ_ROOM_FIXED_GSO |
+ BPF_F_ADJ_ROOM_NO_CSUM_RESET)))
return -EINVAL;

if (skb_is_gso(skb) && !skb_is_gso_tcp(skb)) {
@@ -3174,7 +3175,8 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
u32 off;
int ret;

- if (unlikely(flags & ~BPF_F_ADJ_ROOM_MASK))
+ if (unlikely(flags & ~(BPF_F_ADJ_ROOM_MASK |
+ BPF_F_ADJ_ROOM_NO_CSUM_RESET)))
return -EINVAL;
if (unlikely(len_diff_abs > 0xfffU))
return -EFAULT;
@@ -3202,6 +3204,8 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,

ret = shrink ? bpf_skb_net_shrink(skb, off, len_diff_abs, flags) :
bpf_skb_net_grow(skb, off, len_diff_abs, flags);
+ if (!ret && !(flags & BPF_F_ADJ_ROOM_NO_CSUM_RESET))
+ __skb_reset_checksum_unnecessary(skb);

bpf_compute_data_pointers(skb);
return ret;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 7bbf1b65be10..ad77cf9bb37e 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1589,6 +1589,13 @@ union bpf_attr {
* Grow or shrink the room for data in the packet associated to
* *skb* by *len_diff*, and according to the selected *mode*.
*
+ * By default, the helper will reset any offloaded checksum
+ * indicator of the skb to CHECKSUM_NONE. This can be avoided
+ * by the following flag:
+ *
+ * * **BPF_F_ADJ_ROOM_NO_CSUM_RESET**: Do not reset offloaded
+ * checksum data of the skb to CHECKSUM_NONE.
+ *
* There are two supported modes at this time:
*
* * **BPF_ADJ_ROOM_MAC**: Adjust room at the mac layer
@@ -3235,6 +3242,7 @@ enum {
BPF_F_ADJ_ROOM_ENCAP_L3_IPV6 = (1ULL << 2),
BPF_F_ADJ_ROOM_ENCAP_L4_GRE = (1ULL << 3),
BPF_F_ADJ_ROOM_ENCAP_L4_UDP = (1ULL << 4),
+ BPF_F_ADJ_ROOM_NO_CSUM_RESET = (1ULL << 5),
};

enum {
--
2.25.1