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Since commit30f241fcf5("xsk: Fix immature cq descriptor production"), the descriptor number is stored in skb control block and xsk_cq_submit_addr_locked() relies on it to put the umem addrs onto pool's completion queue. skb control block shouldn't be used for this purpose as after transmit xsk doesn't have control over it and other subsystems could use it. This leads to the following kernel panic due to a NULL pointer dereference. BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 0 P4D 0 Oops: Oops: 0000 [#1] SMP NOPTI 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 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014 RIP: 0010:xsk_destruct_skb+0xd0/0x180 [...] Call Trace: <IRQ> ? napi_complete_done+0x7a/0x1a0 ip_rcv_core+0x1bb/0x340 ip_rcv+0x30/0x1f0 __netif_receive_skb_one_core+0x85/0xa0 process_backlog+0x87/0x130 __napi_poll+0x28/0x180 net_rx_action+0x339/0x420 handle_softirqs+0xdc/0x320 ? handle_edge_irq+0x90/0x1e0 do_softirq.part.0+0x3b/0x60 </IRQ> <TASK> __local_bh_enable_ip+0x60/0x70 __dev_direct_xmit+0x14e/0x1f0 __xsk_generic_xmit+0x482/0xb70 ? __remove_hrtimer+0x41/0xa0 ? __xsk_generic_xmit+0x51/0xb70 ? _raw_spin_unlock_irqrestore+0xe/0x40 xsk_sendmsg+0xda/0x1c0 __sys_sendto+0x1ee/0x200 __x64_sys_sendto+0x24/0x30 do_syscall_64+0x84/0x2f0 ? __pfx_pollwake+0x10/0x10 ? __rseq_handle_notify_resume+0xad/0x4c0 ? restore_fpregs_from_fpstate+0x3c/0x90 ? switch_fpu_return+0x5b/0xe0 ? do_syscall_64+0x204/0x2f0 ? do_syscall_64+0x204/0x2f0 ? do_syscall_64+0x204/0x2f0 entry_SYSCALL_64_after_hwframe+0x76/0x7e </TASK> [...] Kernel panic - not syncing: Fatal exception in interrupt Kernel Offset: 0x1c000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff) Instead use the skb destructor_arg pointer along with pointer tagging. As pointers are always aligned to 8B, use the bottom bit to indicate whether this a single address or an allocated struct containing several addresses. Fixes:30f241fcf5("xsk: Fix immature cq descriptor production") Closes: https://lore.kernel.org/netdev/0435b904-f44f-48f8-afb0-68868474bf1c@nop.hu/ Suggested-by: Jakub Kicinski <kuba@kernel.org> Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de> Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com> Reviewed-by: Jason Xing <kerneljasonxing@gmail.com> Link: https://patch.msgid.link/20251124171409.3845-1-fmancera@suse.de Signed-off-by: Jakub Kicinski <kuba@kernel.org>