Re: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference

Rusty Russell
2008-06-24T13:16:47Z

On Tuesday 24 June 2008 18:06:23 Zhang, Yanmin wrote:
> On Tue, 2008-06-24 at 11:36 +1000, Rusty Russell wrote:
> > On Tuesday 24 June 2008 02:58:44 Mike Travis wrote:
> > > Rusty Russell wrote:
> > > > On Monday 23 June 2008 02:29:07 Vegard Nossum wrote:
> > > >> And the (cpu < nr_cpu_ids) fails because the CPU has just been
> > > >> offlined (or failed to initialize, but it's the same thing), while
> > > >> NR_CPUS is the value that was compiled in as CONFIG_NR_CPUS (so the
> > > >> former check will always be true).
> > > >>
> > > >> I don't think it is valid to ask for a per_cpu() variable on a CPU
> > > >> which does not exist, though
> > > >
> > > > Yes it is. As long as cpu_possible(cpu), per_cpu(cpu) is valid.
> > > >
> > > > The number check should be removed: checking cpu_possible() is
> > > > sufficient.
> > > >
> > > > Hope that helps,
> > > > Rusty.
> > >
> > > I don't see a check for index being out of range in cpu_possible().
> >
> > You're right. It assumes cpu is < NR_CPUS. Hmm, I have no idea what's
> > going on. nr_cpu_ids (ignore that it's a horrible name for a bad idea)
> > should be fine to test against.
> >
> > Vegard's analysis is flawed: just because cpu is offline, it still must
> > be < nr_cpu_ids, which is based on possible cpus. Unless something crazy
> > is happening, but a quick grep doesn't reveal anyone manipulating
> > nr_cpu_ids.
> >
> > If changing this fixes the bug, something else is badly wrong...
> > Rusty.
>
> In function _cpu_up, the panic happens when calling
> __raw_notifier_call_chain at the second time. Kernel doesn't panic when
> calling it at the first time. If just say because ﻿of nr_cpu_ids, that's
> not right.
>
> By checking source codes, I find function do_boot_cpu is the culprit.
> Consider below call chain:
> _cpu_up=>__cpu_up=>smp_ops.cpu_up=>native_cpu_up=>do_boot_cpu.
>
> So ﻿do_boot_cpu is called in the end. In ﻿do_boot_cpu, if boot_error==true,
> cpu_clear(cpu, cpu_possible_map) is executed. So later on, when ﻿_cpu_up
> calls ﻿__raw_notifier_call_chain at the second time to report
> CPU_UP_CANCELED, because this cpu is already cleared from
> ﻿cpu_possible_map, get_cpu_sysdev returns NULL.
>
> Many resources are related to ﻿cpu_possible_map, so it's better not to
> change it.
>
> Below patch against 2.6.26-rc7 fixes it by removing the bit clearing in
> ﻿cpu_possible_map.
>
> Vegard, would you like to help test it?
>
> ﻿﻿﻿Signed-off-by: Zhang Yanmin ﻿<yanmin_zhang@linux.intel.com>
>
> ---
>
> diff -Nraup linux-2.6.26-rc7/arch/x86/kernel/smpboot.c
> linux-2.6.26-rc7_cpuhotplug/arch/x86/kernel/smpboot.c ---
> linux-2.6.26-rc7/arch/x86/kernel/smpboot.c 2008-06-24 09:03:54.000000000
> +0800 +++ linux-2.6.26-rc7_cpuhotplug/arch/x86/kernel/smpboot.c 2008-06-24
> 09:04:45.000000000 +0800 @@ -996,7 +996,6 @@ do_rest:
> #endif
> cpu_clear(cpu, cpu_callout_map); /* was set by do_boot_cpu() */
> cpu_clear(cpu, cpu_initialized); /* was set by cpu_init() */
> - cpu_clear(cpu, cpu_possible_map);
> cpu_clear(cpu, cpu_present_map);
> per_cpu(x86_cpu_to_apicid, cpu) = BAD_APICID;
> }

Nice catch. Basically, cpu_possible_map should only be cleared at boot, and
probably not even then.

Acked-by: Rusty Russell <rusty@rustcorp.com.au>

Thanks,
Rusty.