From: Jakub Sitnicki jakub@cloudflare.com
[ Upstream commit d51c5907e9809a803b276883d203f45849abd4d6 ]
GSO expects inner transport header offset to be valid when skb->encapsulation flag is set. GSO uses this value to calculate the length of an individual segment of a GSO packet in skb_gso_transport_seglen().
However, tcp/udp gro_complete callbacks don't update the skb->inner_transport_header when processing an encapsulated TCP/UDP segment. As a result a GRO skb has ->inner_transport_header set to a value carried over from earlier skb processing.
This can have mild to tragic consequences. From miscalculating the GSO segment length to triggering a page fault [1], when trying to read TCP/UDP header at an address past the skb->data page.
The latter scenario leads to an oops report like so:
BUG: unable to handle page fault for address: ffff9fa7ec00d008 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 123f201067 P4D 123f201067 PUD 123f209067 PMD 0 Oops: 0000 [#1] SMP NOPTI CPU: 44 PID: 0 Comm: swapper/44 Not tainted 5.4.53-cloudflare-2020.7.21 #1 Hardware name: HYVE EDGE-METAL-GEN10/HS-1811DLite1, BIOS V2.15 02/21/2020 RIP: 0010:skb_gso_transport_seglen+0x44/0xa0 Code: c0 41 83 e0 11 f6 87 81 00 00 00 20 74 30 0f b7 87 aa 00 00 00 0f [...] RSP: 0018:ffffad8640bacbb8 EFLAGS: 00010202 RAX: 000000000000feda RBX: ffff9fcc8d31bc00 RCX: ffff9fa7ec00cffc RDX: ffff9fa7ebffdec0 RSI: 000000000000feda RDI: 0000000000000122 RBP: 00000000000005c4 R08: 0000000000000001 R09: 0000000000000000 R10: ffff9fe588ae3800 R11: ffff9fe011fc92f0 R12: ffff9fcc8d31bc00 R13: ffff9fe0119d4300 R14: 00000000000005c4 R15: ffff9fba57d70900 FS: 0000000000000000(0000) GS:ffff9fe68df00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: ffff9fa7ec00d008 CR3: 0000003e99b1c000 CR4: 0000000000340ee0 Call Trace: <IRQ> skb_gso_validate_network_len+0x11/0x70 __ip_finish_output+0x109/0x1c0 ip_sublist_rcv_finish+0x57/0x70 ip_sublist_rcv+0x2aa/0x2d0 ? ip_rcv_finish_core.constprop.0+0x390/0x390 ip_list_rcv+0x12b/0x14f __netif_receive_skb_list_core+0x2a9/0x2d0 netif_receive_skb_list_internal+0x1b5/0x2e0 napi_complete_done+0x93/0x140 veth_poll+0xc0/0x19f [veth] ? mlx5e_napi_poll+0x221/0x610 [mlx5_core] net_rx_action+0x1f8/0x790 __do_softirq+0xe1/0x2bf irq_exit+0x8e/0xc0 do_IRQ+0x58/0xe0 common_interrupt+0xf/0xf </IRQ>
The bug can be observed in a simple setup where we send IP/GRE/IP/TCP packets into a netns over a veth pair. Inside the netns, packets are forwarded to dummy device:
trafgen -> [veth A]--[veth B] -forward-> [dummy]
For veth B to GRO aggregate packets on receive, it needs to have an XDP program attached (for example, a trivial XDP_PASS). Additionally, for UDP, we need to enable GSO_UDP_L4 feature on the device:
ip netns exec A ethtool -K AB rx-udp-gro-forwarding on
The last component is an artificial delay to increase the chances of GRO batching happening:
ip netns exec A tc qdisc add dev AB root \ netem delay 200us slot 5ms 10ms packets 2 bytes 64k
With such a setup in place, the bug can be observed by tracing the skb outer and inner offsets when GSO skb is transmitted from the dummy device:
tcp:
FUNC DEV SKB_LEN NH TH ENC INH ITH GSO_SIZE GSO_TYPE ip_finish_output dumB 2830 270 290 1 294 254 1383 (tcpv4,gre,) ^^^ udp:
FUNC DEV SKB_LEN NH TH ENC INH ITH GSO_SIZE GSO_TYPE ip_finish_output dumB 2818 270 290 1 294 254 1383 (gre,udp_l4,) ^^^
Fix it by updating the inner transport header offset in tcp/udp gro_complete callbacks, similar to how {inet,ipv6}_gro_complete callbacks update the inner network header offset, when skb->encapsulation flag is set.
[1] https://lore.kernel.org/netdev/CAKxSbF01cLpZem2GFaUaifh0S-5WYViZemTicAg7FCHO...
Fixes: bf296b125b21 ("tcp: Add GRO support") Fixes: f993bc25e519 ("net: core: handle encapsulation offloads when computing segment lengths") Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.") Reported-by: Alex Forster aforster@cloudflare.com Signed-off-by: Jakub Sitnicki jakub@cloudflare.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_offload.c | 3 +++ net/ipv4/udp_offload.c | 4 ++++ 2 files changed, 7 insertions(+)
diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c index e09147ac9a99..fc61cd3fea65 100644 --- a/net/ipv4/tcp_offload.c +++ b/net/ipv4/tcp_offload.c @@ -298,6 +298,9 @@ int tcp_gro_complete(struct sk_buff *skb) if (th->cwr) skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
+ if (skb->encapsulation) + skb->inner_transport_header = skb->transport_header; + return 0; } EXPORT_SYMBOL(tcp_gro_complete); diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c index 6e2b02cf7841..f4b8e56068e0 100644 --- a/net/ipv4/udp_offload.c +++ b/net/ipv4/udp_offload.c @@ -615,6 +615,10 @@ static int udp_gro_complete_segment(struct sk_buff *skb)
skb_shinfo(skb)->gso_segs = NAPI_GRO_CB(skb)->count; skb_shinfo(skb)->gso_type |= SKB_GSO_UDP_L4; + + if (skb->encapsulation) + skb->inner_transport_header = skb->transport_header; + return 0; }