[PATCH v6 bpf-next 0/2] Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap

[PATCH v6 bpf-next 0/2] Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap

[Feng Yang]

From: Feng Yang <yangfeng@kylinos.cn>

Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap

Changes in v6:
- Modify the bpf_lwt_xmit_push_encap function and add selftests for it.
  Thanks, Martin KaFai Lau.
- Link to v5: https://lore.kernel.org/all/20260210090657.86977-1-yangfeng59949@163.com/
Changes in v5:
- Refer to the bpf_lwt_xmit_reroute function to configure the dst parameter.
- Link to v4: https://lore.kernel.org/all/20260209015111.28144-1-yangfeng59949@163.com/
Changes in v4:
- add rcu lock
- Link to v3: https://lore.kernel.org/all/20260206055113.63476-1-yangfeng59949@163.com/
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/ 

Feng Yang (2):
  bpf: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap
  selftests/bpf: Add selftests for the invocation of
    bpf_lwt_xmit_push_encap

 net/core/lwt_bpf.c                            |  7 +++++--
 .../selftests/bpf/progs/verifier_lwt.c        | 20 +++++++++++++++++++
 2 files changed, 25 insertions(+), 2 deletions(-)

-- 
2.43.0

[PATCH v6 bpf-next 1/2] bpf: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap

[Feng Yang]

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

Check the skb->_skb_refdst in bpf_lwt_push_ip_encap function, and return
-EINVAL if _skb_refdst is NULL.

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/core/lwt_bpf.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index 9f40be0c3e71..374054e1a2c6 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -616,11 +616,14 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
 		return -EINVAL;
 	}
 
-	if (ingress)
+	if (ingress) {
 		err = skb_cow_head(skb, len + skb->mac_len);
-	else
+	} else {
+		if (unlikely(!skb_dst(skb)))
+			return -EINVAL;
 		err = skb_cow_head(skb,
 				   len + LL_RESERVED_SPACE(skb_dst(skb)->dev));
+	}
 	if (unlikely(err))
 		return err;
 
-- 
2.43.0

Re: [PATCH v6 bpf-next 1/2] bpf: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap

[Martin KaFai Lau]

On 2/12/26 6:44 PM, 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
> 
> Check the skb->_skb_refdst in bpf_lwt_push_ip_encap function, and return
> -EINVAL if _skb_refdst is NULL.
> 
> 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/core/lwt_bpf.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
> index 9f40be0c3e71..374054e1a2c6 100644
> --- a/net/core/lwt_bpf.c
> +++ b/net/core/lwt_bpf.c
> @@ -616,11 +616,14 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
>   		return -EINVAL;
>   	}
>   
> -	if (ingress)
> +	if (ingress) {
>   		err = skb_cow_head(skb, len + skb->mac_len);
> -	else
> +	} else {
> +		if (unlikely(!skb_dst(skb)))

Please check if the v5 discussion make sense (fix in test_run.c). It 
should be something simpler than v5, e.g. always assign a dst to skb for 
is_lwt case. I think always using net->ipv6.ip6_null_entry should work 
but it needs to check ipv6 mod is loaded/enabled before using. Checking 
'ipv6_bpf_stub' should do. I don't think we need to complicate things 
for v4 only kernel in test_run. I don't think testing skb->protocol 
matters also from looking at bpf_lwt_push_ip_encap but please check.

pw-bot: cr

> +			return -EINVAL;
>   		err = skb_cow_head(skb,
>   				   len + LL_RESERVED_SPACE(skb_dst(skb)->dev));
> +	}
>   	if (unlikely(err))
>   		return err;
>   

Re: [PATCH v6 bpf-next 1/2] bpf: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap

[Feng Yang]

On Mon, 23 Feb 2026 13:39:51 -0800, Martin KaFai Lau wrote:

...
> Please check if the v5 discussion make sense (fix in test_run.c). It 
> should be something simpler than v5, e.g. always assign a dst to skb for 
> is_lwt case. I think always using net->ipv6.ip6_null_entry should work 
> but it needs to check ipv6 mod is loaded/enabled before using. Checking 
> 'ipv6_bpf_stub' should do. I don't think we need to complicate things 
> for v4 only kernel in test_run. I don't think testing skb->protocol 
> matters also from looking at bpf_lwt_push_ip_encap but please check.


Sure, I'll check, modify, and test it. Thanks.

[PATCH v6 bpf-next 2/2] selftests/bpf: Add selftests for the invocation of bpf_lwt_xmit_push_encap

[Feng Yang]

From: Feng Yang <yangfeng@kylinos.cn>

Calling bpf_lwt_xmit_push_encap will not cause a crash when dst is missing.

Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
---
 .../selftests/bpf/progs/verifier_lwt.c        | 20 +++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_lwt.c b/tools/testing/selftests/bpf/progs/verifier_lwt.c
index 5ab746307309..9cef07e6eceb 100644
--- a/tools/testing/selftests/bpf/progs/verifier_lwt.c
+++ b/tools/testing/selftests/bpf/progs/verifier_lwt.c
@@ -231,4 +231,24 @@ __naked void not_permitted_for_lwt_prog(void)
 	: __clobber_all);
 }
 
+SEC("lwt_xmit")
+__description("missing dst when calling bpf_lwt_xmit_push_encap")
+__success __retval(0)
+__naked void missing_dst_call_bpf_lwt_xmit_push_encap(void)
+{
+	asm volatile ("					\
+	r9 = 69;					\
+	r3 = r10;					\
+	r3 += -32;					\
+	*(u64 *)(r3 + 0) = r9;				\
+	r2 = 2;						\
+	r4 = 20;					\
+	call %[bpf_lwt_push_encap];			\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_lwt_push_encap)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0