Re: [PATCH v8 bpf-next 1/2] bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap

[Martin KaFai Lau <martin.lau@linux.dev>]

On 2/27/26 12:21 AM, Feng Yang wrote:
> From: Feng Yang <yangfeng@kylinos.cn>
> 
> 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]  <TASK>
> [  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 <dddddd@hust.edu.cn>
> Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
> Closes: https://groups.google.com/g/hust-os-kernel-patches/c/8-a0kPpBW2s
> Signed-off-by: Yun Lu <luyun@kylinos.cn>
> Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
> Tested-by: syzbot@syzkaller.appspotmail.com
> ---
>   net/bpf/test_run.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 178c4738e63b..54dbda2f6329 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -1156,6 +1156,19 @@ 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 && !skb_dst(skb)) {

Has skb_dst ever been set earlier?
If not, can the "!skb_dst" test be skipped?

> +		if (!ipv6_bpf_stub) {
> +			pr_warn_once("Please test this program with the IPv6 module enabled\n");
> +			ret = -EOPNOTSUPP;
> +			goto out;
> +		}
> +#if IS_ENABLED(CONFIG_IPV6)
> +		rcu_read_lock();
> +		skb_dst_set_noref(skb, &net->ipv6.ip6_null_entry->dst);
> +		rcu_read_unlock();
> +#endif

Although ip6_null_entry will not be freed, this still does not
look right to use it after rcu_read_unlock(). Does it have issue to
just take the dst_hold() first and then skb_dst_set()?

Also move all changes into the IS_ENABLED(CONFIG_IPV6). Something like this:

	if (prog->type == BPF_PROG_TYPE_LWT_XMIT) {
		if (IS_ENABLED(CONFIG_IPV6) && ipv6_bpf_struct) {
			dst_hold(&net->ipv6.ip6_null_entry->dst);
			skb_dst_set(skb, &net->ipv6.ip6_null_entry->dst);
		} else {
			pr_warn_once("Please test this program with the IPv6 module enabled\n");
			ret = -EOPNOTSUPP;
			goto out;
		}
	}

pw-bot: cr

> +	}
> +
>   	ret = bpf_test_run(prog, skb, repeat, &retval, &duration, false);
>   	if (ret)
>   		goto out;