| Message ID | 20200210161046.221258-1-toke@redhat.com |
|---|---|
| State | Accepted |
| Delegated to: | David Miller |
| Headers | show |
| Series | [net] core: Don't skip generic XDP program execution for cloned SKBs | expand |
From: Toke Høiland-Jørgensen <toke@redhat.com> Date: Mon, 10 Feb 2020 17:10:46 +0100 > The current generic XDP handler skips execution of XDP programs entirely if > an SKB is marked as cloned. This leads to some surprising behaviour, as > packets can end up being cloned in various ways, which will make an XDP > program not see all the traffic on an interface. Yeah this has been a sort spot for a while, applied and queued up for -stable, thanks.
diff --git a/net/core/dev.c b/net/core/dev.c index a69e8bd7ed74..a6316b336128 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -4527,14 +4527,14 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb, /* Reinjected packets coming from act_mirred or similar should * not get XDP generic processing. */ - if (skb_cloned(skb) || skb_is_tc_redirected(skb)) + if (skb_is_tc_redirected(skb)) return XDP_PASS; /* XDP packets must be linear and must have sufficient headroom * of XDP_PACKET_HEADROOM bytes. This is the guarantee that also * native XDP provides, thus we need to do it here as well. */ - if (skb_is_nonlinear(skb) || + if (skb_cloned(skb) || skb_is_nonlinear(skb) || skb_headroom(skb) < XDP_PACKET_HEADROOM) { int hroom = XDP_PACKET_HEADROOM - skb_headroom(skb); int troom = skb->tail + skb->data_len - skb->end;
The current generic XDP handler skips execution of XDP programs entirely if an SKB is marked as cloned. This leads to some surprising behaviour, as packets can end up being cloned in various ways, which will make an XDP program not see all the traffic on an interface. This was discovered by a simple test case where an XDP program that always returns XDP_DROP is installed on a veth device. When combining this with the Scapy packet sniffer (which uses an AF_PACKET) socket on the sending side, SKBs reliably end up in the cloned state, causing them to be passed through to the receiving interface instead of being dropped. A minimal reproducer script for this is included below. This patch fixed the issue by simply triggering the existing linearisation code for cloned SKBs instead of skipping the XDP program execution. This behaviour is in line with the behaviour of the native XDP implementation for the veth driver, which will reallocate and copy the SKB data if the SKB is marked as shared. Reproducer Python script (requires BCC and Scapy): from scapy.all import TCP, IP, Ether, sendp, sniff, AsyncSniffer, Raw, UDP from bcc import BPF import time, sys, subprocess, shlex SKB_MODE = (1 << 1) DRV_MODE = (1 << 2) PYTHON=sys.executable def client(): time.sleep(2) # Sniffing on the sender causes skb_cloned() to be set s = AsyncSniffer() s.start() for p in range(10): sendp(Ether(dst="aa:aa:aa:aa:aa:aa", src="cc:cc:cc:cc:cc:cc")/IP()/UDP()/Raw("Test"), verbose=False) time.sleep(0.1) s.stop() return 0 def server(mode): prog = BPF(text="int dummy_drop(struct xdp_md *ctx) {return XDP_DROP;}") func = prog.load_func("dummy_drop", BPF.XDP) prog.attach_xdp("a_to_b", func, mode) time.sleep(1) s = sniff(iface="a_to_b", count=10, timeout=15) if len(s): print(f"Got {len(s)} packets - should have gotten 0") return 1 else: print("Got no packets - as expected") return 0 if len(sys.argv) < 2: print(f"Usage: {sys.argv[0]} <skb|drv>") sys.exit(1) if sys.argv[1] == "client": sys.exit(client()) elif sys.argv[1] == "server": mode = SKB_MODE if sys.argv[2] == 'skb' else DRV_MODE sys.exit(server(mode)) else: try: mode = sys.argv[1] if mode not in ('skb', 'drv'): print(f"Usage: {sys.argv[0]} <skb|drv>") sys.exit(1) print(f"Running in {mode} mode") for cmd in [ 'ip netns add netns_a', 'ip netns add netns_b', 'ip -n netns_a link add a_to_b type veth peer name b_to_a netns netns_b', # Disable ipv6 to make sure there's no address autoconf traffic 'ip netns exec netns_a sysctl -qw net.ipv6.conf.a_to_b.disable_ipv6=1', 'ip netns exec netns_b sysctl -qw net.ipv6.conf.b_to_a.disable_ipv6=1', 'ip -n netns_a link set dev a_to_b address aa:aa:aa:aa:aa:aa', 'ip -n netns_b link set dev b_to_a address cc:cc:cc:cc:cc:cc', 'ip -n netns_a link set dev a_to_b up', 'ip -n netns_b link set dev b_to_a up']: subprocess.check_call(shlex.split(cmd)) server = subprocess.Popen(shlex.split(f"ip netns exec netns_a {PYTHON} {sys.argv[0]} server {mode}")) client = subprocess.Popen(shlex.split(f"ip netns exec netns_b {PYTHON} {sys.argv[0]} client")) client.wait() server.wait() sys.exit(server.returncode) finally: subprocess.run(shlex.split("ip netns delete netns_a")) subprocess.run(shlex.split("ip netns delete netns_b")) Fixes: d445516966dc ("net: xdp: support xdp generic on virtual devices") Reported-by: Stepan Horacek <shoracek@redhat.com> Suggested-by: Paolo Abeni <pabeni@redhat.com> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com> --- net/core/dev.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)