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

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Message-Id: <200806242314.51656.rusty@rustcorp.com.au>
Date:	Tue, 24 Jun 2008 23:14:51 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Mike Travis <travis@....com>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Adrian Bunk <bunk@...nel.org>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	linux-kernel@...r.kernel.org, Gautham R Shenoy <ego@...ibm.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference

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@...ux.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@...tcorp.com.au>

Thanks,
Rusty.
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