Re: [PATCH v5] bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap

[Feng Yang]

On Tue, 10 Feb 2026 11:10:03 -0800 Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:

> On 2/10/26 1:06 AM, Feng Yang wrote:
> > From: Feng Yang <yangfeng@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxx>
> > Reported-by: Kaiyan Mei <M202472210@xxxxxxxxxxx>
> > Closes: https://groups.google.com/g/hust-os-kernel-patches/c/8-a0kPpBW2s
> > Signed-off-by: Yun Lu <luyun@xxxxxxxxxx>
> > Signed-off-by: Feng Yang <yangfeng@xxxxxxxxxx>
> > Tested-by: syzbot@xxxxxxxxxxxxxxxxxxxxxxxxx
> > ---
> > 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@xxxxxxx/
> > Changes in v4:
> > - add rcu lock
> > - Link to v3: https://lore.kernel.org/all/20260206055113.63476-1-yangfeng59949@xxxxxxx/
> > Changes in v3:
> > - use dst_init
> > - Link to v2: https://lore.kernel.org/all/20260205092227.126665-1-yangfeng59949@xxxxxxx/
> > Changes in v2:
> > - Link to v1: https://lore.kernel.org/all/20260127084520.13890-1-luyun_611@xxxxxxx/
> 
> The earlier syzbot reports are crying for a selftest which is still 
> missing in v5.
> 
> The CI has also reported errors in the test_progs. Did you run any test 
> before posting ?

My apologies. I only tested whether a crash would occur 
when using `bpf_prog_test_run_skb` to execute `bpf_lwt_push_ip_encap` without a dst entry.
I will include a selftest in my next submission.

> pw-bot: cr
> 
> > ---
> >   net/bpf/test_run.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 47 insertions(+)
> > 
> > diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> > index 178c4738e63b..dbd2c44da7e5 100644
> > --- a/net/bpf/test_run.c
> > +++ b/net/bpf/test_run.c
> > @@ -24,6 +24,7 @@
> >   #include <net/netdev_rx_queue.h>
> >   #include <net/xdp.h>
> >   #include <net/netfilter/nf_bpf_link.h>
> > +#include <net/ip6_route.h>
> >   
> >   #define CREATE_TRACE_POINTS
> >   #include <trace/events/bpf_test_run.h>
> > @@ -992,6 +993,7 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr,
> >   	u32 headroom = NET_SKB_PAD + NET_IP_ALIGN;
> >   	u32 linear_sz = kattr->test.data_size_in;
> >   	u32 repeat = kattr->test.repeat;
> > +	struct dst_entry *dst = NULL;
> >   	struct __sk_buff *ctx = NULL;
> >   	struct sk_buff *skb = NULL;
> >   	struct sock *sk = NULL;
> > @@ -1156,6 +1158,51 @@ 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)) {
> > +		struct flowi4 fl4 = {};
> > +		struct flowi6 fl6 = {};
> > +		struct rtable *rt;
> > +
> > +		switch (skb->protocol) {
> > +		case htons(ETH_P_IP):
> > +			if (sizeof(struct iphdr) <= skb_headlen(skb)) {
> > +				fl4.saddr = ip_hdr(skb)->saddr;
> > +				fl4.daddr = ip_hdr(skb)->daddr;
> > +			}
> > +
> > +			rt = ip_route_output_key(net, &fl4);
> 
> What can be expected from the return value rt if fl4 is 0.

If it is empty, `ip_route_output_key_hash_rcu` will assign the loopback address,
and the returned `rt` is valid, not an error value.

> > +			if (IS_ERR(rt)) {
> > +				ret = PTR_ERR(rt);
> 
> I suspect this is probably what failed in CI. Add a NULL check in 
> bpf_lwt_push_ip_encap instead. No change is needed in the 
> bpf_prog_test_run_skb.

The root cause of the CI failure is that the default path was taken, resulting in a return value of -EINVAL.
So, is your suggestion to modify the bpf_lwt_push_ip_encap function as in the v1(https://lore.kernel.org/all/20260127084520.13890-1-luyun_611@xxxxxxx/) version?
However, the previous suggestion was to make changes to the bpf_prog_test_run_skb function instead.

Thank you very much for your reply.

> > +				goto out;
> > +			}
> > +			dst = &rt->dst;
> > +			break;
> > +#if IS_ENABLED(CONFIG_IPV6)
> > +		case htons(ETH_P_IPV6):
> > +			if (sizeof(struct ipv6hdr) <= skb_headlen(skb)) {
> > +				fl6.saddr = ipv6_hdr(skb)->saddr;
> > +				fl6.daddr = ipv6_hdr(skb)->daddr;
> > +			}
> > +
> > +			dst = ip6_route_output(net, NULL, &fl6);
> > +			if (IS_ERR(dst)) {
> > +				ret = PTR_ERR(dst);
> > +				goto out;
> > +			}
> > +			break;
> > +#endif
> > +		default:
> > +			ret = -EINVAL;

The reason for the CI test failure.

> > +			goto out;
> > +		}
> > +
> > +		if (unlikely(dst->error)) {
> > +			ret = dst->error;
> > +			dst_release(dst);
> > +			goto out;
> > +		}
> > +		skb_dst_set(skb, dst);
> > +	}
> >   	ret = bpf_test_run(prog, skb, repeat, &retval, &duration, false);
> >   	if (ret)
> >   		goto out;