* [PATCH v7 bpf-next 0/2] Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap
@ 2026-02-26 9:51 Feng Yang
2026-02-26 9:51 ` [PATCH v7 bpf-next 1/2] bpf: test_run: " Feng Yang
2026-02-26 9:51 ` [PATCH v7 bpf-next 2/2] selftests/bpf: Add selftests for the invocation of bpf_lwt_xmit_push_encap Feng Yang
0 siblings, 2 replies; 4+ messages in thread
From: Feng Yang @ 2026-02-26 9:51 UTC (permalink / raw)
To: davem, edumazet, kuba, pabeni, horms, posk, ast, daniel, andrii,
martin.lau, eddyz87
Cc: bpf, netdev, linux-kernel, linux-kselftest, yangfeng59949
From: Feng Yang <yangfeng@kylinos.cn>
Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap
Changes in v7:
- Use ip6_null_entry to avoid. Thanks, Martin KaFai Lau.
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: test_run: 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/bpf/test_run.c | 12 ++++++++++++
.../selftests/bpf/progs/verifier_lwt.c | 19 +++++++++++++++++++
2 files changed, 31 insertions(+)
--
2.43.0
^ permalink raw reply [flat|nested] 4+ messages in thread
* [PATCH v7 bpf-next 1/2] bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap
2026-02-26 9:51 [PATCH v7 bpf-next 0/2] Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap Feng Yang
@ 2026-02-26 9:51 ` Feng Yang
2026-02-26 10:41 ` bot+bpf-ci
2026-02-26 9:51 ` [PATCH v7 bpf-next 2/2] selftests/bpf: Add selftests for the invocation of bpf_lwt_xmit_push_encap Feng Yang
1 sibling, 1 reply; 4+ messages in thread
From: Feng Yang @ 2026-02-26 9:51 UTC (permalink / raw)
To: davem, edumazet, kuba, pabeni, horms, posk, ast, daniel, andrii,
martin.lau, eddyz87
Cc: bpf, netdev, linux-kernel, linux-kselftest, yangfeng59949
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 | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 178c4738e63b..9ee2dc09eeb6 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -1156,6 +1156,18 @@ 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)) {
+ if (!ipv6_bpf_stub) {
+ pr_warn_once("Please test this program with the IPv6 module enabled\n");
+ 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
+ }
+
ret = bpf_test_run(prog, skb, repeat, &retval, &duration, false);
if (ret)
goto out;
--
2.43.0
^ permalink raw reply related [flat|nested] 4+ messages in thread
* [PATCH v7 bpf-next 2/2] selftests/bpf: Add selftests for the invocation of bpf_lwt_xmit_push_encap
2026-02-26 9:51 [PATCH v7 bpf-next 0/2] Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap Feng Yang
2026-02-26 9:51 ` [PATCH v7 bpf-next 1/2] bpf: test_run: " Feng Yang
@ 2026-02-26 9:51 ` Feng Yang
1 sibling, 0 replies; 4+ messages in thread
From: Feng Yang @ 2026-02-26 9:51 UTC (permalink / raw)
To: davem, edumazet, kuba, pabeni, horms, posk, ast, daniel, andrii,
martin.lau, eddyz87
Cc: bpf, netdev, linux-kernel, linux-kselftest, yangfeng59949
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 | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
diff --git a/tools/testing/selftests/bpf/progs/verifier_lwt.c b/tools/testing/selftests/bpf/progs/verifier_lwt.c
index 5ab746307309..a2b45e3e1542 100644
--- a/tools/testing/selftests/bpf/progs/verifier_lwt.c
+++ b/tools/testing/selftests/bpf/progs/verifier_lwt.c
@@ -231,4 +231,23 @@ __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]; \
+ exit; \
+" :
+ : __imm(bpf_lwt_push_encap)
+ : __clobber_all);
+}
+
char _license[] SEC("license") = "GPL";
--
2.43.0
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH v7 bpf-next 1/2] bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap
2026-02-26 9:51 ` [PATCH v7 bpf-next 1/2] bpf: test_run: " Feng Yang
@ 2026-02-26 10:41 ` bot+bpf-ci
0 siblings, 0 replies; 4+ messages in thread
From: bot+bpf-ci @ 2026-02-26 10:41 UTC (permalink / raw)
To: yangfeng59949, davem, edumazet, kuba, pabeni, horms, posk, ast,
daniel, andrii, martin.lau, eddyz87
Cc: bpf, netdev, linux-kernel, linux-kselftest, yangfeng59949, ast,
andrii, daniel, martin.lau, eddyz87, yonghong.song, clm,
ihor.solodrai
[-- Attachment #1: Type: text/plain, Size: 1623 bytes --]
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -1156,6 +1156,18 @@
> skb->ip_summed = CHECKSUM_COMPLETE;
> }
>
> + if (prog->type == BPF_PROG_TYPE_LWT_XMIT && !skb_dst(skb)) {
> + if (!ipv6_bpf_stub) {
> + pr_warn_once("Please test this program with the IPv6 module enabled\n");
> + goto out;
Should this set ret to an error code before goto out? At this point
ret is 0 from the earlier convert___skb_to_skb() success path, so the
function returns 0 to the bpf() syscall without ever running the BPF
program.
Every other early exit in bpf_prog_test_run_skb() assigns a negative
errno before goto out, for example:
ret = -EINVAL;
goto out;
ret = -ENOMEM;
goto out;
ret = -ENODEV;
goto out;
Returning 0 here means userspace sees a successful test run that never
actually executed. Something like ret = -EOPNOTSUPP would let the
caller know the test could not proceed.
> + }
> [ ... ]
---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/22437468613
AI-authorship-score: low
AI-authorship-explanation: Patch iterated through 7 versions over 3 weeks with detailed maintainer interaction, consistent with human development.
issues-found: 1
issue-severity-score: low
issue-severity-explanation: Missing error return code on early exit path causes bpf() syscall to return success without executing the BPF program when IPv6 is unavailable.
^ permalink raw reply [flat|nested] 4+ messages in thread
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2026-02-26 9:51 [PATCH v7 bpf-next 0/2] Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap Feng Yang
2026-02-26 9:51 ` [PATCH v7 bpf-next 1/2] bpf: test_run: " Feng Yang
2026-02-26 10:41 ` bot+bpf-ci
2026-02-26 9:51 ` [PATCH v7 bpf-next 2/2] selftests/bpf: Add selftests for the invocation of bpf_lwt_xmit_push_encap Feng Yang
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