Re: [syzbot] [kernel?] upstream test error: KMSAN: uninit-value in irqentry_exit_to_kernel_mode_preempt

[Dmitry Vyukov]

On Tue, 21 Apr 2026 at 23:37, syzbot
<syzbot+cdcfd5...@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 4ee64205ffaa Merge tag 'clk-for-linus' of git://git.kernel..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1706d702580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=ce4d707ce513c810
> dashboard link: https://syzkaller.appspot.com/bug?extid=cdcfd55737fe43eeb3a3
> compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/eed48de63104/disk-4ee64205.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/6ebe911e6995/vmlinux-4ee64205.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/557a239d8722/bzImage-4ee64205.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+cdcfd5...@syzkaller.appspotmail.com
>

+kmsan maintainers, not sure where it should be fixed: in irq code or
in kmsan code

> =====================================================
> BUG: KMSAN: uninit-value in irqentry_exit_to_kernel_mode_preempt+0xb0/0xc0 include/linux/irq-entry-common.h:472
> irqentry_exit_to_kernel_mode_preempt+0xb0/0xc0 include/linux/irq-entry-common.h:472
> irqentry_exit_to_kernel_mode include/linux/irq-entry-common.h:547 [inline]
> irqentry_exit+0x7b/0x760 kernel/entry/common.c:164
> sysvec_apic_timer_interrupt+0x52/0x90 arch/x86/kernel/apic/apic.c:1061
> asm_sysvec_apic_timer_interrupt+0x1f/0x30 arch/x86/include/asm/idtentry.h:697
> __sanitizer_cov_trace_pc+0x5/0x70 kernel/kcov.c:210
> nsim_dev_trap_skb_build drivers/net/netdevsim/dev.c:842 [inline]
> nsim_dev_trap_report drivers/net/netdevsim/dev.c:876 [inline]
> nsim_dev_trap_report_work+0x8c0/0x1430 drivers/net/netdevsim/dev.c:922
> process_one_work kernel/workqueue.c:3302 [inline]
> process_scheduled_works+0xb65/0x1e40 kernel/workqueue.c:3385
> worker_thread+0xee4/0x1590 kernel/workqueue.c:3466
> kthread+0x53f/0x600 kernel/kthread.c:436
> ret_from_fork+0x20f/0x8d0 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>
> Uninit was created at:
> slab_post_alloc_hook mm/slub.c:4576 [inline]
> slab_alloc_node mm/slub.c:4898 [inline]
> __do_kmalloc_node mm/slub.c:5294 [inline]
> __kmalloc_node_track_caller_noprof+0x4f6/0x1750 mm/slub.c:5403
> kmalloc_reserve net/core/skbuff.c:635 [inline]
> __alloc_skb+0x90d/0x1190 net/core/skbuff.c:713
> alloc_skb include/linux/skbuff.h:1383 [inline]
> nsim_dev_trap_skb_build drivers/net/netdevsim/dev.c:819 [inline]
> nsim_dev_trap_report drivers/net/netdevsim/dev.c:876 [inline]
> nsim_dev_trap_report_work+0x3f2/0x1430 drivers/net/netdevsim/dev.c:922
> process_one_work kernel/workqueue.c:3302 [inline]
> process_scheduled_works+0xb65/0x1e40 kernel/workqueue.c:3385
> worker_thread+0xee4/0x1590 kernel/workqueue.c:3466
> kthread+0x53f/0x600 kernel/kthread.c:436
> ret_from_fork+0x20f/0x8d0 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>
> CPU: 1 UID: 0 PID: 13 Comm: kworker/u8:1 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
> Workqueue: events_unbound nsim_dev_trap_report_work
> =====================================================
>
>
> ---
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[Alexander Potapenko]

On Wed, Apr 29, 2026 at 8:29 AM Dmitry Vyukov <dvy...@google.com> wrote:
>
> On Tue, 21 Apr 2026 at 23:37, syzbot
> <syzbot+cdcfd5...@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit: 4ee64205ffaa Merge tag 'clk-for-linus' of git://git.kernel..
> > git tree: upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1706d702580000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=ce4d707ce513c810
> > dashboard link: https://syzkaller.appspot.com/bug?extid=cdcfd55737fe43eeb3a3
> > compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/eed48de63104/disk-4ee64205.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/6ebe911e6995/vmlinux-4ee64205.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/557a239d8722/bzImage-4ee64205.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+cdcfd5...@syzkaller.appspotmail.com
> >
>
> +kmsan maintainers, not sure where it should be fixed: in irq code or
> in kmsan code

I believe Mark's recent changes introduced this.
irqentry_exit_to_kernel_mode_preempt() is now checking for both `regs`
and `state`.
Because there is a lot of non-instrumented code around, we fail to
initialize these variables.
Instead, irqentry_enter_from_kernel_mode() explicitly calls
kmsan_unpoison_entry_regs(regs) to take care of the registers, but not
the state.
We should probably call `kmsan_unpoison_memory(&state, sizeof(state))`
at the same place.