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

Gautham R Shenoy
2008-06-26T13:10:51Z

On Tue, Jun 24, 2008 at 11:14:51PM +1000, Rusty Russell wrote:
> 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);

Nice catch.

While we're at it, is the clearing of cpu from the cpu_present_map
necessary if cpu_up failed for 'cpu' ?

> > 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.

--
Thanks and Regards
gautham