Re: [syzbot] [kernel?] KMSAN: kernel-infoleak in exc_page_fault

[Thomas Gleixner]

#syz set subsystems: kasan

On Wed, Jun 24 2026 at 18:08, syzbot wrote:
> syzbot found the following issue on:
>
> HEAD commit:    9ecfb2f7287a Merge tag 'trace-ring-buffer-v7.2' of git://g..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1471e8ea580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=a9e1383387f0e112
> dashboard link: https://syzkaller.appspot.com/bug?extid=411634dace73dad6f4d4
> compiler:       Debian clang version 22.1.6 (++20260514074242+fc4aad7b5db3-1~exp1~20260514074407.73), Debian LLD 22.1.6
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/c8d222bd96ba/disk-9ecfb2f7.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/629368d40b7e/vmlinux-9ecfb2f7.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/156086f5f12a/bzImage-9ecfb2f7.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+411634dace73dad6f4d4@xxxxxxxxxxxxxxxxxxxxxxxxx

As I already explained here:

   https://lore.kernel.org/871pe1ng7m.ffs@fw13

this is a KMSAN/compiler bug:

> BUG: KMSAN: kernel-infoleak in rseq_set_ids_get_csaddr include/linux/rseq_entry.h:502 [inline]
> BUG: KMSAN: kernel-infoleak in rseq_update_usr include/linux/rseq_entry.h:536 [inline]
> BUG: KMSAN: kernel-infoleak in rseq_exit_user_update include/linux/rseq_entry.h:645 [inline]
> BUG: KMSAN: kernel-infoleak in __rseq_exit_to_user_mode_restart include/linux/rseq_entry.h:674 [inline]
> BUG: KMSAN: kernel-infoleak in rseq_exit_to_user_mode_restart include/linux/rseq_entry.h:703 [inline]
> BUG: KMSAN: kernel-infoleak in exit_to_user_mode_loop kernel/entry/common.c:103 [inline]
> BUG: KMSAN: kernel-infoleak in __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
> BUG: KMSAN: kernel-infoleak in irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:244 [inline]
> BUG: KMSAN: kernel-infoleak in irqentry_exit_to_user_mode include/linux/irq-entry-common.h:315 [inline]
> BUG: KMSAN: kernel-infoleak in irqentry_exit+0x4a6/0xa40 kernel/entry/common.c:165
>  exc_page_fault+0x7e/0xb0 arch/x86/mm/fault.c:1539
>  asm_exc_page_fault+0x2b/0x30 arch/x86/include/asm/idtentry.h:595

There was a real issue before 6d99479799c6 got applied, but KMSAN still
claims that a local variable which is completely unrelated and was
created in a (previous) syscall is the problem:

> Local variable st.i.i created at:
>  __do_sys_statfs fs/statfs.c:193 [inline]
>  __se_sys_statfs fs/statfs.c:191 [inline]
>  __x64_sys_statfs+0x73/0x200 fs/statfs.c:191
>  x64_sys_call+0x334c/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:138

That's clearly some stale information.

> Bytes 0-3 of 4 are uninitialized
> Memory access of size 4 starts at ffff88811856be78
> Data copied to user address 00007ff99a04cac0

The local variable access in rseq_set_ids_get_csaddr() is
ids->cpu_id. ids is a stack variable created _and_ correctly initialized
in rseq_exit_user_update().