linux-kernel - Re: [PATCH] [workqueue] check values of pwq and wq in print_worker

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Message-ID: <BLU0-SMTP14B0EBB3853FE8B06192DB97160@phx.gbl>
Date:	Tue, 1 Oct 2013 21:58:23 -0400
From:	John David Anglin <dave.anglin@...l.net>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	Helge Deller <deller@....de>, Tejun Heo <tj@...nel.org>,
	Libin <huawei.libin@...wei.com>, linux-kernel@...r.kernel.org,
	linux-parisc@...r.kernel.org
Subject: Re: [PATCH] [workqueue] check values of pwq and wq in print_worker_info() before use

On 1-Oct-13, at 6:50 PM, James Bottomley wrote:

> On Wed, 2013-10-02 at 00:07 +0200, Helge Deller wrote:
>> On 10/01/2013 11:40 PM, James Bottomley wrote:
>>> On Tue, 2013-10-01 at 16:43 -0400, Tejun Heo wrote:
>>>> Hello,
>>>>
>>>> On Tue, Oct 01, 2013 at 10:35:20PM +0200, Helge Deller wrote:
>>>>> print_worker_info() includes no validity check on the pwq and wq
>>>>> pointers before handing them over to the probe_kernel_read()  
>>>>> functions.
>>>>>
>>>>> It seems that most architectures don't care about that, but at  
>>>>> least on
>>>>> the parisc architecture this leads to a kernel crash since  
>>>>> accesses to
>>>>> page zero are protected by the kernel for security reasons.
>>>>>
>>>>> Fix this problem by verifying the contents of pwq and wq before  
>>>>> usage.
>>>>> Even if probe_kernel_read() usually prevents such crashes by  
>>>>> disabling
>>>>> page faults, clean code should always include such checks.
>>>>>
>>>>> Without this fix issuing "echo t > /proc/sysrq-trigger" will  
>>>>> immediately
>>>>> crash the Linux kernel on the parisc architecture.
>>>>
>>>> Hmm... um had similar problem but the root cause here is that the  
>>>> arch
>>>> isn't implementing probe_kernel_read() properly.  We really have no
>>>> idea what the pointer value may be at the dump point and that's  
>>>> why we
>>>> use probe_kernel_read().  If something like the above is  
>>>> necessary for
>>>> the time being, the correct place would be the arch
>>>> probe_kernel_read() implementation.  James, would it be difficult
>>>> implement proper probe_kernel_read() on parisc?
>>>
>>> The problem seems to be that some traps bypass our exception table
>>> handling.
>>
>> Yes, that's correct.
>> It's trap #26 and we directly call parisc_terminate() for  
>> fault_space==0
>> without checking the exception table.
>> See my patch I posted a few hours ago which fixes this:
>> https://patchwork.kernel.org/patch/2971701/
>
> That doesn't quite look right ... I guessed it was probably access
> rights, so we should do an exception table fixup, so isn't this the  
> fix?
> because we shouldn't call do_page_fault if there's no exception table.
>
> James
>
> ---
> diff --git a/arch/parisc/kernel/traps.c b/arch/parisc/kernel/traps.c
> index 04e47c6..25a088a 100644
> --- a/arch/parisc/kernel/traps.c
> +++ b/arch/parisc/kernel/traps.c
> @@ -684,6 +684,8 @@ void notrace handle_interruption(int code,  
> struct pt_regs *regs)
> 		/* Fall Through */
> 	case 26:
> 		/* PCXL: Data memory access rights trap */
> +		if (!user_mode(regs) && fixup_exception(regs))
> +			return;
> 		fault_address = regs->ior;
> 		fault_space   = regs->isr;
> 		break;


With this change, boot on rp3440 hangs here:

Freeing unused kernel memory: 256K (000000004079c000 - 00000000407dc000)
Loading, please wait...

Dave
--
John David Anglin	dave.anglin@...l.net



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