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

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Message-ID: <CANn89i+8hX9SjbhR2GOe+RfEkeksKCtPbkz-6pQhCA=pjnr5zg@mail.gmail.com>
Date: Wed, 17 Dec 2025 14:49:49 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Aditya Gupta <adityag@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [REPORT] Null pointer deref in net/core/dev.c on PowerPC

On Wed, Dec 17, 2025 at 1:10 PM Aditya Gupta <adityag@...ux.ibm.com> wrote:
>
> 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
>

This seems to be a platform issue.

prefetch(NULL) (or prefetch (amount < PAGE_SIZE)) is not supposed to fault.