netdev - Re: [PATCH net v3] xsk: avoid data corruption on cq descriptor number

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Message-ID: <CAL+tcoBx88qdt5BXjEvRUh1bxW2-Q9PGMjRcEjotAEzw1=hkVQ@mail.gmail.com>
Date: Fri, 31 Oct 2025 18:23:42 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Fernando Fernandez Mancera <fmancera@...e.de>
Cc: netdev@...r.kernel.org, csmate@....hu, maciej.fijalkowski@...el.com, 
	bjorn@...nel.org, sdf@...ichev.me, jonathan.lemon@...il.com, 
	bpf@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com, 
	kuba@...nel.org, pabeni@...hat.com, horms@...nel.org
Subject: Re: [PATCH net v3] xsk: avoid data corruption on cq descriptor number

On Fri, Oct 31, 2025 at 6:05 PM Fernando Fernandez Mancera
<fmancera@...e.de> wrote:
>
>
>
> On 10/31/25 10:51 AM, Jason Xing wrote:
> > On Thu, Oct 30, 2025 at 10:04 PM Fernando Fernandez Mancera
> > <fmancera@...e.de> 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)
> >>
> >> 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: 30f241fcf52a ("xsk: Fix immature cq descriptor production")
> >> Closes: https://lore.kernel.org/netdev/0435b904-f44f-48f8-afb0-68868474bf1c@nop.hu/
> >> Suggested-by: Jakub Kicinski <kuba@...nel.org>
> >> Signed-off-by: Fernando Fernandez Mancera <fmancera@...e.de>
> >
> > I don't think we need this fix anymore if we can apply the series[1].
> > The fix I just proposed doesn't use any new bits to store something so
> > the problem will disappear.
> >
> > [1]: https://lore.kernel.org/all/20251031093230.82386-1-kerneljasonxing@gmail.com/
> >
> > Thanks,
> > Jason
> >
>
> Right. Then let's consider this patch dropped.

Only if we all agree on that new approach :P Any suggestions are welcome :)

Thanks,
Jason