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

Zhang, Yanmin
2008-06-24T08:08:29Z

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;
}