bpf: lwt: Free the skb when bpf_xmit() fails to expand the headroom

When an LWT xmit program returns BPF_OK, bpf_xmit() makes sure that
the skb still has room for the link layer header. If
pskb_expand_head() fails there, bpf_xmit() returns -ENOMEM without
freeing the skb. Its callers, lwtunnel_xmit() and then
ip_finish_output2() or ip6_finish_output2(), take any return value
other than LWTUNNEL_XMIT_CONTINUE to mean that the skb has been
consumed, so nothing frees it.

Each such failure leaks the skb, together with what it holds: its dst
and its charge to the sending socket, so the socket is never freed
either. The other error paths of bpf_xmit() (BPF_DROP, an illegal
return code, a changed protocol, a failed reroute) all free the skb.

Free it here too.

The leak was found with kbound, a static analysis prototype for
ownership mismatches at kernel interfaces, on v7.2.12; it is present in
mainline.

Build-tested with x86_64 and arm64 allmodconfig (W=1, sparse): no new
warnings. Run-tested on QEMU (x86_64, KVM) with KASAN: a route through a
dummy device with an LWT xmit program that pushes one byte with
bpf_skb_change_head(), so the headroom check has to expand the head, and
failslab limited to allocations under bpf_xmit(). Each of 10 pings then
fails with ENOMEM. Without this patch each leaves a raw socket behind
(RAW slab objects 6 -> 16); with it none does (6 -> 6), and nothing else
is reported. Without injection both kernels go from 6 to 6.

Fixes: 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure")
Signed-off-by: Yuqiang Sun <izaiahsun@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261101182343.17991-1-izaiahsun@gmail.com

0 files changed, 0 insertions, 0 deletions