Re: [RFC 2/2] mm: harden alloc_pages code paths against bogus nodes

[Michal Hocko]

On Wed 01-08-18 15:04:18, Jeremy Linton wrote:
> Its possible to crash __alloc_pages_nodemask by passing it
> bogus node ids. This is caused by NODE_DATA() returning null
> (hopefully) when the requested node is offline. We can
> harded against the basic case of a mostly valid node, that
> isn't online by checking for null and failing prepare_alloc_pages.
> 
> But this then suggests we should also harden NODE_DATA() like this
> 
> #define NODE_DATA(nid)         ( (nid) < MAX_NUMNODES ? node_data[(nid)] : NULL)
> 
> eventually this starts to add a bunch of generally uneeded checks
> in some code paths that are called quite frequently.

But the page allocator is really a hot path and people will not be happy
to have yet another branch there. No code should really use invalid numa
node ids in the first place.

If I remember those bugs correctly then it was the arch code which was
doing something wrong. I would prefer that code to be fixed instead.

> Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
> ---
>  include/linux/gfp.h | 2 ++
>  mm/page_alloc.c     | 2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index a6afcec53795..17d70271c42e 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -436,6 +436,8 @@ static inline int gfp_zonelist(gfp_t flags)
>   */
>  static inline struct zonelist *node_zonelist(int nid, gfp_t flags)
>  {
> +	if (unlikely(!NODE_DATA(nid))) //VM_WARN_ON?
> +		return NULL;
>  	return NODE_DATA(nid)->node_zonelists + gfp_zonelist(flags);
>  }
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a790ef4be74e..3a3d9ac2662a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4306,6 +4306,8 @@ static inline bool prepare_alloc_pages(gfp_t gfp_mask, unsigned int order,
>  {
>  	ac->high_zoneidx = gfp_zone(gfp_mask);
>  	ac->zonelist = node_zonelist(preferred_nid, gfp_mask);
> +	if (!ac->zonelist)
> +		return false;
>  	ac->nodemask = nodemask;
>  	ac->migratetype = gfpflags_to_migratetype(gfp_mask);
>  
> -- 
> 2.14.3
> 

-- 
Michal Hocko
SUSE Labs