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Message-ID: <CAL+tcoA5qDAcnZpmULsnD=X6aVP-ztRxPv5z1OSP-nvtNEk+-w@mail.gmail.com>
Date: Mon, 20 Oct 2025 14:41:47 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: mc36 <csmate@....hu>
Cc: Jonathan Lemon <jonathan.lemon@...il.com>, Stanislav Fomichev <sdf@...ichev.me>,
Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Magnus Karlsson <magnus.karlsson@...el.com>,
Björn Töpel <bjorn@...nel.org>, 1118437@...s.debian.org,
netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: null pointer dereference in interrupt after receiving an ip
packet on veth from xsk from user space
Hi,
On Mon, Oct 20, 2025 at 12:50 PM mc36 <csmate@....hu> wrote:
>
> hi,
>
> freertr.org xsk dataplane is triggering a null pointer de-reference after sending
>
> a single ip packet from user space to an xsk socket bound to a veth interface.
>
> the same code works fine when using physical interfaces to send packets to...
>
> it does not matter if an ip address is assigned to the veth pair or not...
>
> please find below the reproducer code with some comments on how to use it...
>
> tldr: create a veth pair, bring them up, compile and run the c code...
>
> this happens 10/10 on host or in qemu-system-x86_64-kvm running 6.16.12 or 6.17.2...
Thanks for the report.
I'm wondering if you have time to bisect which recent commit has
brought this problem. It looks like it never happens before 6.16?
Thanks,
Jason
>
> if it does not belongs here, just let me know....
>
> have a nice day,
>
> csaba
>
>
> ------------------------------------------------------[cut]---------------------------------------------------------
>
>
> p4emu login: [ 119.074634] BUG: kernel NULL pointer dereference, address: 0000000000000000
> [ 119.076747] #PF: supervisor read access in kernel mode
> [ 119.078334] #PF: error_code(0x0000) - not-present page
> [ 119.079855] PGD 0 P4D 0
> [ 119.080648] Oops: Oops: 0000 [#1] SMP NOPTI
> [ 119.081993] CPU: 2 UID: 1 PID: 927 Comm: p4xsk.bin Not tainted 6.16.12+deb14-cloud-amd64 #1 PREEMPT(lazy) Debian 6.16.12-1
> [ 119.085247] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
> [ 119.088065] RIP: 0010:xsk_destruct_skb+0xd0/0x180
> [ 119.089502] Code: 40 10 48 89 cf 89 28 e8 9e 7e 07 00 48 89 df 48 83 c4 18 5b 5d 41 5c 41 5d 41 5e 41 5f e9 c8 cc da ff 48 8b 7b 30 4c 8d 5b 30 <48> 8b 07 4c 8d 67 f8 4c 8d 70
> f8 49 39 fb 74 b7 48 89 5c 24 10 4c
> [ 119.094947] RSP: 0018:ffffcd5b4012cd48 EFLAGS: 00010002
> [ 119.096499] RAX: ffffcd5b40fcf000 RBX: ffff898e05dfcf00 RCX: ffff898e043cf9e8
> [ 119.098612] RDX: ffff898e048ccc80 RSI: 0000000000000246 RDI: 0000000000000000
> [ 119.100687] RBP: 0000000000000001 R08: 0000000000000000 R09: ffff898e01d21900
> [ 119.102794] R10: 0000000000000000 R11: ffff898e05dfcf30 R12: ffff898e05f95000
> [ 119.104880] R13: ffff898e043cf900 R14: ffff898e7dd32bd0 R15: 0000000000000002
> [ 119.107000] FS: 00007f0cd9e0a6c0(0000) GS:ffff898ede530000(0000) knlGS:0000000000000000
> [ 119.109358] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 119.111080] CR2: 0000000000000000 CR3: 00000000043ba003 CR4: 0000000000372ef0
> [ 119.113175] Call Trace:
> [ 119.113996] <IRQ>
> [ 119.114662] ? napi_complete_done+0x7a/0x1a0
> [ 119.115952] ip_rcv_core+0x1bb/0x340
> [ 119.117050] ip_rcv+0x30/0x1f0
> [ 119.118014] __netif_receive_skb_one_core+0x85/0xa0
> [ 119.119468] process_backlog+0x87/0x130
> [ 119.120617] __napi_poll+0x28/0x180
> [ 119.121685] net_rx_action+0x339/0x420
> [ 119.122850] handle_softirqs+0xdc/0x320
> [ 119.124003] ? handle_edge_irq+0x90/0x1e0
> [ 119.125218] do_softirq.part.0+0x3b/0x60
> [ 119.126422] </IRQ>
> [ 119.127085] <TASK>
> [ 119.127753] __local_bh_enable_ip+0x60/0x70
> [ 119.128998] __dev_direct_xmit+0x14e/0x1f0
> [ 119.130128] __xsk_generic_xmit+0x482/0xb70
> [ 119.131184] ? __remove_hrtimer+0x41/0xa0
> [ 119.132199] ? __xsk_generic_xmit+0x51/0xb70
> [ 119.133300] ? _raw_spin_unlock_irqrestore+0xe/0x40
> [ 119.134637] xsk_sendmsg+0xda/0x1c0
> [ 119.135580] __sys_sendto+0x1ee/0x200
> [ 119.136509] __x64_sys_sendto+0x24/0x30
> [ 119.137493] do_syscall_64+0x84/0x2f0
> [ 119.138452] ? __pfx_pollwake+0x10/0x10
> [ 119.139454] ? __rseq_handle_notify_resume+0xad/0x4c0
> [ 119.140718] ? restore_fpregs_from_fpstate+0x3c/0x90
> [ 119.141999] ? switch_fpu_return+0x5b/0xe0
> [ 119.143023] ? do_syscall_64+0x204/0x2f0
> [ 119.144007] ? do_syscall_64+0x204/0x2f0
> [ 119.144990] ? do_syscall_64+0x204/0x2f0
> [ 119.146022] entry_SYSCALL_64_after_hwframe+0x76/0x7e
> [ 119.147278] RIP: 0033:0x7f0cde0a49ee
> [ 119.148217] Code: 08 0f 85 f5 4b ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> 66 2e 0f 1f 84 00 00 00 00
> 00 0f 1f 80 00 00 00 00 48 83 ec 08
> [ 119.152877] RSP: 002b:00007f0cd9e09c98 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
> [ 119.154774] RAX: ffffffffffffffda RBX: 00007f0cd9e0a6c0 RCX: 00007f0cde0a49ee
> [ 119.156526] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000029
> [ 119.158317] RBP: 0000000000000005 R08: 0000000000000000 R09: 0000000000000000
> [ 119.160078] R10: 0000000000000040 R11: 0000000000000246 R12: 0000000000000405
> [ 119.161893] R13: 00007f0ccc055ce0 R14: 0000000000000001 R15: 00007f0cde8db900
> [ 119.163646] </TASK>
> [ 119.164243] Modules linked in: veth intel_rapl_msr intel_rapl_common iosf_mbi binfmt_misc kvm_intel kvm irqbypass ghash_clmulni_intel sha512_ssse3 sha1_ssse3 aesni_intel rapl
> button evdev sg efi_pstore configfs nfnetlink vsock_loopback vmw_vsock_virtio_transport_common vmw_vsock_vmci_transport vsock vmw_vmci qemu_fw_cfg ip_tables x_tables autofs4 sd_mod
> sr_mod cdrom ata_generic ata_piix libata virtio_net scsi_mod net_failover serio_raw failover scsi_common
> [ 119.174216] CR2: 0000000000000000
> [ 119.175068] ---[ end trace 0000000000000000 ]---
> [ 119.176224] RIP: 0010:xsk_destruct_skb+0xd0/0x180
> [ 119.177432] Code: 40 10 48 89 cf 89 28 e8 9e 7e 07 00 48 89 df 48 83 c4 18 5b 5d 41 5c 41 5d 41 5e 41 5f e9 c8 cc da ff 48 8b 7b 30 4c 8d 5b 30 <48> 8b 07 4c 8d 67 f8 4c 8d 70
> f8 49 39 fb 74 b7 48 89 5c 24 10 4c
> [ 119.182155] RSP: 0018:ffffcd5b4012cd48 EFLAGS: 00010002
> [ 119.183462] RAX: ffffcd5b40fcf000 RBX: ffff898e05dfcf00 RCX: ffff898e043cf9e8
> [ 119.185237] RDX: ffff898e048ccc80 RSI: 0000000000000246 RDI: 0000000000000000
> [ 119.187022] RBP: 0000000000000001 R08: 0000000000000000 R09: ffff898e01d21900
> [ 119.188872] R10: 0000000000000000 R11: ffff898e05dfcf30 R12: ffff898e05f95000
> [ 119.190693] R13: ffff898e043cf900 R14: ffff898e7dd32bd0 R15: 0000000000000002
> [ 119.192655] FS: 00007f0cd9e0a6c0(0000) GS:ffff898ede530000(0000) knlGS:0000000000000000
> [ 119.194681] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 119.196244] CR2: 0000000000000000 CR3: 00000000043ba003 CR4: 0000000000372ef0
> [ 119.198034] Kernel panic - not syncing: Fatal exception in interrupt
> [ 119.199761] Kernel Offset: 0x1c000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
> [ 119.202403] ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---
>
>
> ------------------------------------------------------[cut]---------------------------------------------------------
>
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <string.h>
> #include <pthread.h>
> #include <arpa/inet.h>
> #include <sys/socket.h>
> #include <netinet/in.h>
> #include <poll.h>
> #include <linux/if_link.h>
> #include <xdp/xsk.h>
>
> ////// gcc xskInt.c -lxdp
> ////// sudo ip link add veth1 type veth
> ////// sudo ip link set veth0 up
> ////// sudo ip link set veth1 up
> ////// sudo ./a.out
>
> void err(char*buf) {
> printf("%s\n", buf);
> _exit(1);
> }
>
>
>
>
> int main(int argc, char **argv) {
>
> int bpf_flag = XDP_FLAGS_SKB_MODE;
>
> char ifaceName[] = "veth0";
> printf("opening interface %s\n", ifaceName);
>
> #define framesNum 1024
>
> struct xsk_umem *ifaceUmem;
> struct xsk_socket *ifaceXsk;
> struct xsk_ring_prod ifaceFq;
> struct xsk_ring_cons ifaceCq;
> struct xsk_ring_cons ifaceRx;
> struct xsk_ring_prod ifaceTx;
> char *ifaceBuf;
> struct pollfd ifacePfd;
>
> posix_memalign((void**)&ifaceBuf, getpagesize(), XSK_UMEM__DEFAULT_FRAME_SIZE * 2 * framesNum);
> if (ifaceBuf == NULL) err("error allocating buffer");
>
> if (xsk_umem__create(&ifaceUmem, ifaceBuf, XSK_UMEM__DEFAULT_FRAME_SIZE * 2 * framesNum, &ifaceFq, &ifaceCq, NULL) != 0) err("error creating umem");
>
> struct xsk_socket_config cfg;
> memset(&cfg, 0, sizeof(cfg));
> cfg.rx_size = XSK_RING_CONS__DEFAULT_NUM_DESCS;
> cfg.tx_size = XSK_RING_PROD__DEFAULT_NUM_DESCS;
> cfg.xdp_flags = bpf_flag;
> if (xsk_socket__create(&ifaceXsk, ifaceName, 0, ifaceUmem, &ifaceRx, &ifaceTx, &cfg) != 0) err("error creating xsk");
>
> unsigned int i = 0;
> xsk_ring_prod__reserve(&ifaceFq, framesNum, &i);
> for (i=0; i < framesNum; i++) *xsk_ring_prod__fill_addr(&ifaceFq, i) = i * XSK_UMEM__DEFAULT_FRAME_SIZE;
> xsk_ring_prod__submit(&ifaceFq, framesNum);
>
> memset(&ifacePfd, 0, sizeof (ifacePfd));
> ifacePfd.fd = xsk_socket__fd(ifaceXsk);
> ifacePfd.events = POLLIN | POLLERR;
>
> setgid(1);
> setuid(1);
> printf("serving others\n");
>
>
> unsigned char bufD[] = {0x3a, 0x10, 0x5c, 0x53, 0xb3, 0x5c, 0x0, 0x1e, 0x11, 0x4c, 0x7e, 0x66, 0x8, 0x0, 0x45, 0x0, 0x0, 0x40, 0x0, 0x0, 0x0, 0x0, 0xff, 0x1, 0xb7, 0xa0, 0x1, 0x1,
> 0x1, 0xd, 0x1, 0x1, 0x1, 0xe, 0x8, 0x0, 0xa5, 0xfd, 0x16, 0x3b, 0x3b, 0xc7, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
> 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0};
> int bufS = sizeof(bufD);
>
> unsigned int idx;
> idx = xsk_ring_cons__peek(&ifaceCq, 16, &idx);
> xsk_ring_cons__release(&ifaceCq, idx);
> if (xsk_ring_prod__reserve(&ifaceTx, 1, &idx) < 1) err("error getting index");
> struct xdp_desc *dsc = xsk_ring_prod__tx_desc(&ifaceTx, idx);
> dsc->addr = (framesNum + (idx % framesNum)) * XSK_UMEM__DEFAULT_FRAME_SIZE;
> dsc->options = 0;
> dsc->len = bufS;
> memcpy(ifaceBuf + dsc->addr, bufD, bufS);
> xsk_ring_prod__submit(&ifaceTx, 1);
> if (!xsk_ring_prod__needs_wakeup(&ifaceTx)) err("error waking up");
> sendto(xsk_socket__fd(ifaceXsk), NULL, 0, MSG_DONTWAIT, NULL, 0);
>
> sleep(10);
> }
>
>
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