[PATCH net] bpf: lwt: Free the skb when bpf_xmit() fails to expand the headroom

[Yuqiang Sun <izaiahsun@gmail.com>]

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")
Cc: stable@vger.kernel.org
Signed-off-by: Yuqiang Sun <izaiahsun@gmail.com>
---

Notes:
    The bug was found with kbound, a research static analysis prototype.
    The patch, the changelog and the test setup were written with the help
    of an AI coding assistant (Claude); I have reviewed and tested them as
    described above.
    
    The runs were on mainline d135ab453a74; the code this patch touches is
    the same there and in net, which it is based on.

 net/core/lwt_bpf.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index da49364ec63d..dfb2f9253e81 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -304,8 +304,10 @@ static int bpf_xmit(struct sk_buff *skb)
 			 * made before LWT xmit.
 			 */
 			ret = xmit_check_headroom(skb, hroom);
-			if (unlikely(ret))
+			if (unlikely(ret)) {
+				kfree_skb(skb);
 				return ret;
+			}
 
 			return LWTUNNEL_XMIT_CONTINUE;
 		case BPF_REDIRECT:

base-commit: cb284914e749388eef402938bdb3115fe1f6d337
-- 
2.53.0