Re: [PATCH net v2] xsk: avoid data corruption on cq descriptor number

[Fernando Fernandez Mancera <fmancera@suse.de>]

On 10/24/25 7:16 PM, Maciej Fijalkowski wrote:
> On Fri, Oct 24, 2025 at 12:40:49PM +0200, Fernando Fernandez Mancera wrote:
>> Since commit 30f241fcf52a ("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)
>>
>> The approach proposed stores the first address also in the xsk_addr_node
>> along with the number of descriptors. The head xsk_addr_node is
>> referenced in skb_shinfo(skb)->destructor_arg. The rest of the fragments
>> store the address on the list.
>>
>> This is less efficient as 4 bytes are wasted when storing each address.
> 
> Hi Fernando,
> it's not about 4 bytes being wasted but rather memory allocation that you
> introduce here. I tried hard to avoid hurting non-fragmented traffic,
> below you can find impact reported by Jason from similar approach as
> yours:
> https://lore.kernel.org/bpf/CAL+tcoD=Gn6ZmJ+_Y48vPRyHVHmP-7irsx=fRVRnyhDrpTrEtQ@mail.gmail.com/
> 
> I assume this patch will yield a similar performance degradation...
> 

It does, thank you for explaining Maciej. I have been thinking about 
possible solutions and remembered skb extensions. If I am not wrong, it 
shouldn't yield a performance degratation or at least it should be a 
much less severe one. Although, XDP_SOCKETS Kconfig would require 
"select SKB_EXTENSIONS".

What do you think about this approach? I could draft a series for 
net-next.. I am just looking for different options other than using skb 
control block because I believe similar issues will arise in the future 
even if we fix the current one on ip_rcv..

Thanks,
Fernando.