netdev - Re: [PATCH net v6] xsk: avoid data corruption on cq descriptor number

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Message-ID: <aSXZ37i5CgGKn2RF@boxer>
Date: Tue, 25 Nov 2025 17:31:27 +0100
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Jason Xing <kerneljasonxing@...il.com>
CC: Fernando Fernandez Mancera <fmancera@...e.de>, <netdev@...r.kernel.org>,
	<csmate@....hu>, <bpf@...r.kernel.org>, <davem@...emloft.net>,
	<edumazet@...gle.com>, <kuba@...nel.org>, <pabeni@...hat.com>,
	<horms@...nel.org>, <sdf@...ichev.me>, <hawk@...nel.org>,
	<daniel@...earbox.net>, <ast@...nel.org>, <john.fastabend@...il.com>,
	<magnus.karlsson@...el.com>
Subject: Re: [PATCH net v6] xsk: avoid data corruption on cq descriptor number

On Tue, Nov 25, 2025 at 08:11:37PM +0800, Jason Xing wrote:
> On Tue, Nov 25, 2025 at 7:40 PM Fernando Fernandez Mancera
> <fmancera@...e.de> wrote:
> >
> > On 11/25/25 12:41 AM, Jason Xing wrote:
> > > On Tue, Nov 25, 2025 at 1:14 AM 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>
> > >
> > > Reviewed-by: Jason Xing <kerneljasonxing@...il.com>
> > >
> > > Could you also post a patch on top of net-next as it has diverged from
> > > the net tree?
> > >
> >
> > I think that is handled by maintainers when merging the branches. A
> > repost would be wrong because linux-next.git and linux.git will have a
> > different variant of the same commit..
> 
> But this patch cannot be applied cleanly in the net-next tree...

What we care here is that it applies to net as that's a tree that this
patch has been posted to.
> 
> >
> > Please, let me know if I am wrong here.
> 
> I'm not quite sure either.
> 
> Thanks,
> Jason
> 
> >
> > Thanks,
> > Fernando.