Re: [PATCH] [workqueue] check values of pwq and wq in print_worker_info() before use

[Helge Deller <deller@gmx.de>]

On 10/02/2013 12:50 AM, 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.
>
> 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;

You need to check for preempt_count()!=0 too, which has been increased by pagefault_disable() inside of probe_kernel_read().
Otherwise every simple memcpy(dest,NULL,count) (*) will sucessfully be handled here and we won't trap
on generic invalid memory accesses inside the kernel.

But basically your patch does exactly the same as mine.

Helge

(*) memcpy() uses internally pa_memcpy() which defines the fixup tables.