From: Feng Yang The bpf_lwt_xmit_push_encap helper needs to access skb_dst(skb)->dev to calculate the needed headroom: err = skb_cow_head(skb, len + LL_RESERVED_SPACE(skb_dst(skb)->dev)); But skb->_skb_refdst may not be initialized when the skb is set up by bpf_prog_test_run_skb function. Executing bpf_lwt_push_ip_encap function in this scenario will trigger null pointer dereference, causing a kernel crash as Yinhao reported: [ 105.186365] BUG: kernel NULL pointer dereference, address: 0000000000000000 [ 105.186382] #PF: supervisor read access in kernel mode [ 105.186388] #PF: error_code(0x0000) - not-present page [ 105.186393] PGD 121d3d067 P4D 121d3d067 PUD 106c83067 PMD 0 [ 105.186404] Oops: 0000 [#1] PREEMPT SMP NOPTI [ 105.186412] CPU: 3 PID: 3250 Comm: poc Kdump: loaded Not tainted 6.19.0-rc5 #1 [ 105.186423] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 [ 105.186427] RIP: 0010:bpf_lwt_push_ip_encap+0x1eb/0x520 [ 105.186443] Code: 0f 84 de 01 00 00 0f b7 4a 04 66 85 c9 0f 85 47 01 00 00 31 c0 5b 5d 41 5c 41 5d 41 5e c3 cc cc cc cc 48 8b 73 58 48 83 e6 fe <48> 8b 36 0f b7 be ec 00 00 00 0f b7 b6 e6 00 00 00 01 fe 83 e6 f0 [ 105.186449] RSP: 0018:ffffbb0e0387bc50 EFLAGS: 00010246 [ 105.186455] RAX: 000000000000004e RBX: ffff94c74e036500 RCX: ffff94c74874da00 [ 105.186460] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff94c74e036500 [ 105.186463] RBP: 0000000000000001 R08: 0000000000000002 R09: 0000000000000000 [ 105.186467] R10: ffffbb0e0387bd50 R11: 0000000000000000 R12: ffffbb0e0387bc98 [ 105.186471] R13: 0000000000000014 R14: 0000000000000000 R15: 0000000000000002 [ 105.186484] FS: 00007f166aa4d680(0000) GS:ffff94c8b7780000(0000) knlGS:0000000000000000 [ 105.186490] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 105.186494] CR2: 0000000000000000 CR3: 000000015eade001 CR4: 0000000000770ee0 [ 105.186499] PKRU: 55555554 [ 105.186502] Call Trace: [ 105.186507] [ 105.186513] bpf_lwt_xmit_push_encap+0x2b/0x40 [ 105.186522] bpf_prog_a75eaad51e517912+0x41/0x49 [ 105.186536] ? kvm_clock_get_cycles+0x18/0x30 [ 105.186547] ? ktime_get+0x3c/0xa0 [ 105.186554] bpf_test_run+0x195/0x320 [ 105.186563] ? bpf_test_run+0x10f/0x320 [ 105.186579] bpf_prog_test_run_skb+0x2f5/0x4f0 [ 105.186590] __sys_bpf+0x69c/0xa40 [ 105.186603] __x64_sys_bpf+0x1e/0x30 [ 105.186611] do_syscall_64+0x59/0x110 [ 105.186620] entry_SYSCALL_64_after_hwframe+0x76/0xe0 [ 105.186649] RIP: 0033:0x7f166a97455d Temporarily add the setting of skb->_skb_refdst before bpf_test_run to resolve the issue. Fixes: 52f278774e79 ("bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap") Reported-by: Yinhao Hu Reported-by: Kaiyan Mei Signed-off-by: Yun Lu Signed-off-by: Feng Yang --- Changes in v3: - use dst_init - Link to v2: https://lore.kernel.org/all/20260205092227.126665-1-yangfeng59949@163.com/ Changes in v2: - Link to v1: https://lore.kernel.org/all/20260127084520.13890-1-luyun_611@163.com/ --- net/bpf/test_run.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c index 178c4738e63b..3e1c47d53b82 100644 --- a/net/bpf/test_run.c +++ b/net/bpf/test_run.c @@ -989,6 +989,7 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr, u32 tailroom = SKB_DATA_ALIGN(sizeof(struct skb_shared_info)); struct net *net = current->nsproxy->net_ns; struct net_device *dev = net->loopback_dev; + struct dst_entry bpf_test_run_lwt_xmit_dst; u32 headroom = NET_SKB_PAD + NET_IP_ALIGN; u32 linear_sz = kattr->test.data_size_in; u32 repeat = kattr->test.repeat; @@ -1156,6 +1157,12 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr, skb->ip_summed = CHECKSUM_COMPLETE; } + if (prog->type == BPF_PROG_TYPE_LWT_XMIT) { + dst_init(&bpf_test_run_lwt_xmit_dst, NULL, NULL, + DST_OBSOLETE_NONE, DST_NOCOUNT); + bpf_test_run_lwt_xmit_dst.dev = dev; + skb_dst_set_noref(skb, &bpf_test_run_lwt_xmit_dst); + } ret = bpf_test_run(prog, skb, repeat, &retval, &duration, false); if (ret) goto out; -- 2.43.0