On 4/3/26 17:44, Feng Yang wrote:
> From: Feng Yang <[email protected]>
>
> 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 <[email protected]>
> Reported-by: Kaiyan Mei <[email protected]>
> Closes: https://groups.google.com/g/hust-os-kernel-patches/c/8-a0kPpBW2s
> Signed-off-by: Yun Lu <[email protected]>
> Signed-off-by: Feng Yang <[email protected]>
> Tested-by: [email protected]
> ---
> net/bpf/test_run.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 178c4738e63b..565fe8cbcdc9 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -1156,6 +1156,20 @@ 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) {
> + if (ipv6_bpf_stub) {
> +#if IS_ENABLED(CONFIG_IPV6)
> + dst_hold(&net->ipv6.ip6_null_entry->dst);
> + skb_dst_set(skb, &net->ipv6.ip6_null_entry->dst);
> +#endif
> + } else {
> + /* For CONFIG_IPV6=n, ipv6_bpf_stub is NULL */
> + pr_warn_once("Please test this program with the IPv6
> module enabled\n");
> + ret = -EOPNOTSUPP;
> + goto out;
> + }
> + }
> +
NIT: it would be more readable to handle the error path first.
if (prog->type == BPF_PROG_TYPE_LWT_XMIT) {
if (!ipv6_bpf_stub) {
pr_warn_once("...");
ret = -EOPNOTSUPP;
goto out;
}
#if IS_ENABLED(CONFIG_IPV6)
dst_hold();
skb_dst_set();
#endif
}
Thanks,
Leon
> ret = bpf_test_run(prog, skb, repeat, &retval, &duration, false);
> if (ret)
> goto out;
