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Message-ID: <CANn89iJj_Vyt2g6QewwaNAXAZ+0iso=4yj0t3U11V_nuUk4ThQ@mail.gmail.com>
Date: Wed, 17 Dec 2025 15:41:16 +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 2:58 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Dec 17, 2025 at 2:49 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > 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.
>
> Special casing of NULL was added in :
>
> commit e63f8f439de010b6227c0c9c6f56e2c44dbe5dae
> Author: Olof Johansson <olof@...tin.ibm.com>
> Date: Sat Apr 16 15:24:38 2005 -0700
>
> [PATCH] ppc64: no prefetch for NULL pointers
I will send the following fix, thanks.
diff --git a/net/core/dev.c b/net/core/dev.c
index 9094c0fb8c68..36dc5199037e 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4241,9 +4241,11 @@ static inline int __dev_xmit_skb(struct sk_buff
*skb, struct Qdisc *q,
int count = 0;
llist_for_each_entry_safe(skb, next, ll_list, ll_node) {
- prefetch(next);
- prefetch(&next->priority);
- skb_mark_not_on_list(skb);
+ if (next) {
+ prefetch(next);
+ prefetch(&next->priority);
+ skb_mark_not_on_list(skb);
+ }
rc = dev_qdisc_enqueue(skb, q, &to_free, txq);
count++;
}
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