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

Andrew Morton
2011-08-19T22:53:05Z

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.