Re: [syzbot] [kernel?] WARNING in audit_log_start

[Paul Moore]

On Tue, Sep 3, 2024 at 4:54 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Tue, Sep 03 2024 at 12:24, Kees Cook wrote:
> > On Tue, Sep 03, 2024 at 03:22:17PM -0400, Paul Moore wrote:
> >> > >  might_alloc include/linux/sched/mm.h:337 [inline]
> >> > >  slab_pre_alloc_hook mm/slub.c:3987 [inline]
> >> > >  slab_alloc_node mm/slub.c:4065 [inline]
> >> > >  kmem_cache_alloc_noprof+0x5d/0x2a0 mm/slub.c:4092
> >> > >  audit_buffer_alloc kernel/audit.c:1790 [inline]
> >> > >  audit_log_start+0x15e/0xa30 kernel/audit.c:1912
> >> > >  audit_seccomp+0x63/0x1f0 kernel/auditsc.c:3007
> >>
> >> The audit_seccomp() function allocates an audit buffer using
> >> GFP_KERNEL, which should be the source of the might_sleep.  We can fix
> >> that easily enough by moving to GFP_ATOMIC (either for just this code
> >> path or all callers, need to check that), but I just want to confirm
> >> that we can't sleep here?  I haven't dug into the syscall code in a
> >> while, so I don't recall all the details, but it seems odd to me that
> >> we can't safely sleep here ...
> >
> > I had a similar question.. this is at syscall entry time. What is
> > suddenly different here? We've been doing seccomp logging here for
> > years...
>
> Correct.
>
> syscall_enter_from_user_mode() enables interrupts. At that point
> preempt_count is 0. So after that the task can sleep and schedule.
> Nothing in the call chain leading up to the allocation disables
> preemption or interrupts.
>
> From the actual console log:
>
> do not call blocking ops when !TASK_RUNNING; state=2 set at [<ffffffff81908f9e>] audit_log_start+0x37e/0xa30
>
> I have no idea how that state would leak accross schedule_timeout().

Okay, with no obvious root cause and no reproducer, I'm going to
ignore this for now.  If we start to see this pop up on real systems
and/or syzbot finds a reproducer we can dig into it more.

-- 
paul-moore.com