Re: [PATCH v2] avoid null pointer access in vm_struct

[HAYASAKA Mitsuo <mitsuo.hayasaka.hu@hitachi.com>]

Hi Paul and Andrew,

(2011/08/20 3:53), Paul E. McKenney wrote:
>> @@ -1562,6 +1561,7 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>>  		}
>>  		area->pages[i] = page;
>>  	}
> Don't we need something here to prevent the compiler and/or the CPU
> from reordering the assignment?  Or am I missing how this is otherwise
> prevented?
>

(2011/08/20 7:52), Andrew Morton wrote:
> I think this is still just a workaround to fix up the real bug, and
> that the real bug is that the vm_struct is installed into the vmlist
> *before* it is fully initialised.  It's just wrong to insert an object
> into a globally-visible list and to then start populating it!  If we
> were instead to fully initialise the vm_struct and *then* insert it
> into vmlist, the bug is fixed.
>
> Also I'd agree with Paul's concern regarding cross-CPU memory ordering.
>

I deeply agreed with both of your concern and comments.
I'd like to create the patch where the vm_struct is installed into 
the vmlist *after* it is fully initialized.

Thanks.


(2011/08/20 7:52), Andrew Morton wrote:
> On Fri, 19 Aug 2011 19:51:33 +0900
> Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com> wrote:
> 
>> The /proc/vmallocinfo shows information about vmalloc allocations in vmlist
>> that is a linklist of vm_struct. It, however, may access pages field of
>> vm_struct where a page was not allocated, which results in a null pointer
>> access and leads to a kernel panic.
>>
>> Why this happen:
>> In __vmalloc_area_node(), the nr_pages field of vm_struct are set to the
>> expected number of pages to be allocated, before the actual pages
>> allocations. At the same time, when the /proc/vmallocinfo is read, it
>> accesses the pages field of vm_struct according to the nr_pages field at
>> show_numa_info(). Thus, a null pointer access happens.
>>
>> Patch:
>> This patch sets nr_pages field of vm_struct AFTER the pages allocations
>> finished in __vmalloc_area_node(). So, it can avoid accessing the pages
>> field with unallocated page when show_numa_info() is called.
> 
> I think this is still just a workaround to fix up the real bug, and
> that the real bug is that the vm_struct is installed into the vmlist
> *before* it is fully initialised.  It's just wrong to insert an object
> into a globally-visible list and to then start populating it!  If we
> were instead to fully initialise the vm_struct and *then* insert it
> into vmlist, the bug is fixed.
> 
> Also I'd agree with Paul's concern regarding cross-CPU memory ordering.
>