[PATCH v2] avoid null pointer access in vm_struct

[PATCH v2] avoid null pointer access in vm_struct

[Mitsuo Hayasaka]

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.

Signed-off-by: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---

 mm/vmalloc.c |   10 +++++-----
 1 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 7ef0903..49d8aed 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1529,7 +1529,6 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 	nr_pages = (area->size - PAGE_SIZE) >> PAGE_SHIFT;
 	array_size = (nr_pages * sizeof(struct page *));
 
-	area->nr_pages = nr_pages;
 	/* Please note that the recursion is strictly bounded. */
 	if (array_size > PAGE_SIZE) {
 		pages = __vmalloc_node(array_size, 1, nested_gfp|__GFP_HIGHMEM,
@@ -1538,15 +1537,15 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 	} else {
 		pages = kmalloc_node(array_size, nested_gfp, node);
 	}
-	area->pages = pages;
-	area->caller = caller;
-	if (!area->pages) {
+	if (!pages) {
 		remove_vm_area(area->addr);
 		kfree(area);
 		return NULL;
 	}
+	area->pages = pages;
+	area->caller = caller;
 
-	for (i = 0; i < area->nr_pages; i++) {
+	for (i = 0; i < nr_pages; i++) {
 		struct page *page;
 		gfp_t tmp_mask = gfp_mask | __GFP_NOWARN;
 
@@ -1562,6 +1561,7 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 		}
 		area->pages[i] = page;
 	}
+	area->nr_pages = nr_pages;
 
 	if (map_vm_area(area, prot, &pages))
 		goto fail;

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

[Paul E. McKenney]

On Fri, Aug 19, 2011 at 07:51:33PM +0900, Mitsuo Hayasaka 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.

One question below...

> Signed-off-by: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Namhyung Kim <namhyung@gmail.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
> ---
> 
>  mm/vmalloc.c |   10 +++++-----
>  1 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 7ef0903..49d8aed 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1529,7 +1529,6 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  	nr_pages = (area->size - PAGE_SIZE) >> PAGE_SHIFT;
>  	array_size = (nr_pages * sizeof(struct page *));
> 
> -	area->nr_pages = nr_pages;
>  	/* Please note that the recursion is strictly bounded. */
>  	if (array_size > PAGE_SIZE) {
>  		pages = __vmalloc_node(array_size, 1, nested_gfp|__GFP_HIGHMEM,
> @@ -1538,15 +1537,15 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  	} else {
>  		pages = kmalloc_node(array_size, nested_gfp, node);
>  	}
> -	area->pages = pages;
> -	area->caller = caller;
> -	if (!area->pages) {
> +	if (!pages) {
>  		remove_vm_area(area->addr);
>  		kfree(area);
>  		return NULL;
>  	}
> +	area->pages = pages;
> +	area->caller = caller;
> 
> -	for (i = 0; i < area->nr_pages; i++) {
> +	for (i = 0; i < nr_pages; i++) {
>  		struct page *page;
>  		gfp_t tmp_mask = gfp_mask | __GFP_NOWARN;
> 
> @@ -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?

> +	area->nr_pages = nr_pages;
> 
>  	if (map_vm_area(area, prot, &pages))
>  		goto fail;
> 

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

[Andrew Morton]

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.

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

[HAYASAKA Mitsuo]

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.
> 

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