linux-kernel - Re: UBSAN: Undefined behaviour in mm/page

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Date:   Fri, 9 Nov 2018 21:52:55 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Kyungtae Kim <kt0755@...il.com>, akpm@...ux-foundation.org,
        pavel.tatashin@...rosoft.com, vbabka@...e.cz, osalvador@...e.de,
        rppt@...ux.vnet.ibm.com, aaron.lu@...el.com,
        iamjoonsoo.kim@....com, alexander.h.duyck@...ux.intel.com,
        mgorman@...hsingularity.net, lifeasageek@...il.com,
        threeearcat@...il.com, syzkaller@...glegroups.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Subject: Re: UBSAN: Undefined behaviour in mm/page_alloc.c

On Fri, Nov 09, 2018 at 10:56:04AM +0100, Michal Hocko wrote:
> On Fri 09-11-18 18:41:53, Tetsuo Handa wrote:
> > On 2018/11/09 17:43, Michal Hocko wrote:
> > > @@ -4364,6 +4353,17 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
> > >  	gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
> > >  	struct alloc_context ac = { };
> > >  
> > > +	/*
> > > +	 * In the slowpath, we sanity check order to avoid ever trying to
> > 
> > Please keep the comment up to dated.
> 
> Does this following look better?
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9fc10a1029cf..bf9aecba4222 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4354,10 +4354,8 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
>  	struct alloc_context ac = { };
>  
>  	/*
> -	 * In the slowpath, we sanity check order to avoid ever trying to
> -	 * reclaim >= MAX_ORDER areas which will never succeed. Callers may
> -	 * be using allocators in order of preference for an area that is
> -	 * too large.
> +	 * There are several places where we assume that the order value is sane
> +	 * so bail out early if the request is out of bound.
>  	 */
>  	if (order >= MAX_ORDER) {
>  		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));

if (unlikely()) might help

> 
> > I don't like that comments in OOM code is outdated.
> > 
> > > +	 * reclaim >= MAX_ORDER areas which will never succeed. Callers may
> > > +	 * be using allocators in order of preference for an area that is
> > > +	 * too large.
> > > +	 */
> > > +	if (order >= MAX_ORDER) {
> > 
> > Also, why not to add BUG_ON(gfp_mask & __GFP_NOFAIL); here?
> 
> Because we do not want to blow up the kernel just because of a stupid
> usage of the allocator. Can you think of an example where it would
> actually make any sense?
> 
> I would argue that such a theoretical abuse would blow up on an
> unchecked NULL ptr access. Isn't that enough?
> -- 

Balbir Singh.