linux-kernel - [REPORT] Null pointer deref in net/core/dev.c on PowerPC

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Message-ID: <q3j7p3zkhipxleesykpfrfhznasqnn6mnfqlcphponzvsyavxf@a6ko6obdpso3>
Date: Wed, 17 Dec 2025 17:40:47 +0530
From: Aditya Gupta <adityag@...ux.ibm.com>
To: Eric Dumazet <edumazet@...gle.com>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: [REPORT] Null pointer deref in net/core/dev.c on PowerPC

Hello,

I see a null pointer dereference in 'net/core/dev.c', with 6.19.0-rc1,
when using e1000e device in qemu.

I am able to reproduce the issue on PowerNV and PSeries machines on Power
architecture, though this might be possible on other architectures also.

Console log
-----------

	...
	Starting network: udhcpc: started, v1.35.0
	udhcpc: broadcasting discover
	[    6.389648] Kernel attempted to read user page (0) - exploit attempt? (uid: 0)
	[    6.394166] BUG: Kernel NULL pointer dereference on read at 0x00000000
	[    6.394262] Faulting instruction address: 0xc00000000166e080
	[    6.395253] Oops: Kernel access of bad area, sig: 11 [#1]
	[    6.398372] LE PAGE_SIZE=64K MMU=Radix  SMP NR_CPUS=2048 NUMA pSeries
	[    6.398647] Modules linked in:
	[    6.399553] CPU: 0 UID: 0 PID: 203 Comm: udhcpc Not tainted 6.19.0-rc1+ #3 PREEMPT(voluntary)
	[    6.399757] Hardware name: IBM pSeries (emulated by qemu) POWER9 (architected) 0x4e1202 0xf000005 of:SLOF,git-6b6c16 pSeries
	[    6.400002] NIP:  c00000000166e080 LR: c00000000166e080 CTR: 0000000000000000
	[    6.400148] REGS: c00000000c67b4f0 TRAP: 0300   Not tainted  (6.19.0-rc1+)
	[    6.400275] MSR:  8000000000009033 <SF,EE,ME,IR,DR,RI,LE>  CR: 44022860  XER: 20040147
	[    6.400544] CFAR: c00000000165ef0c DAR: 0000000000000000 DSISR: 40000000 IRQMASK: 0
	[    6.400544] GPR00: c00000000166e080 c00000000c67b790 c0000000028ca300 0000000000000002
	[    6.400544] GPR04: c00000000324a568 000000000001a560 0000000000000020 0000000000000000
	[    6.400544] GPR08: 0000000000000000 0000000000000000 0000000000000201 0000000028022862
	[    6.400544] GPR12: 0000000000000001 c0000000041a0000 0000000000000000 0000000000000000
	[    6.400544] GPR16: 0000000000000000 0000000000000010 0000000000000148 0000000000000148
	[    6.400544] GPR20: 0000000000000000 0000000000000008 00000000000005dc c000000003ea5e98
	[    6.400544] GPR24: c000000003ea5e94 0000000000000000 c000000005b7e200 0000000000000001
	[    6.400544] GPR28: 0000000000000000 0000000000000000 0000000000000000 c000000003ea5d80
	[    6.401178] NIP [c00000000166e080] __dev_xmit_skb+0x484/0xb88
	[    6.401697] LR [c00000000166e080] __dev_xmit_skb+0x484/0xb88
	[    6.401843] Call Trace:
	[    6.401938] [c00000000c67b790] [c00000000166e080] __dev_xmit_skb+0x484/0xb88 (unreliable)
	[    6.402060] [c00000000c67b810] [c0000000016738a4] __dev_queue_xmit+0x4b4/0xa94
	[    6.402122] [c00000000c67b970] [c00000000192748c] packet_xmit+0x10c/0x1b0
	[    6.402190] [c00000000c67b9f0] [c00000000192af6c] packet_snd+0x784/0xa04
	[    6.402278] [c00000000c67bad0] [c00000000162a91c] __sys_sendto+0x1dc/0x250
	[    6.402340] [c00000000c67bc20] [c00000000162a9c4] sys_sendto+0x34/0x44
	[    6.402400] [c00000000c67bc40] [c000000000031870] system_call_exception+0x170/0x360
	[    6.402468] [c00000000c67be50] [c00000000000cedc] system_call_vectored_common+0x15c/0x2ec
	...

Git Blame
---------

Debugging with GDB points to this code in 'net/core/dev.c':

	static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
					 struct net_device *dev,
					 struct netdev_queue *txq)
	{
	...
			llist_for_each_entry_safe(skb, next, ll_list, ll_node) {
				prefetch(next);
				prefetch(&next->priority);						<----------
				skb_mark_not_on_list(skb);
				rc = dev_qdisc_enqueue(skb, q, &to_free, txq);
				count++;
			}

Git blame points to this commit which introduced the use of 'next->priority':

	commit b2e9821cff6c3c9ac107fce5327070f4462bf8a7
	Date:   Fri Nov 21 08:32:52 2025 +0000

	    net: prefech skb->priority in __dev_xmit_skb()

Reproducing the issue
---------------------

To reproduce the issue:
1. Attaching config as attachment
2. Kernel commit I built: 'commit 40fbbd64bba6 ("Merge tag 'pull-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs")'
3. Initramfs (it's buildroot): https://ibm.box.com/s/x70ducx9cxl9tz4abh97d9b508ildync
4. QEMU command line: 'qemu-system-ppc64 -M pseries -m 10G -kernel ~/some-path/zImage -append "init=/bin/sh noreboot debug" -nographic -initrd ~/some-path/rootfs-with-ssh.cpio -netdev user,id=net0 -device e1000e,netdev=net0

Thanks,
- Aditya G


View attachment "config" of type "text/plain" (157201 bytes)