linux-kernel - Re: [PATCH v2 1/4] mm/pgtable: Fix continue to preallocate pmds even if failure occurrence

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Message-Id: <20130820160418.5639c4f9975b84dc8dede014@linux-foundation.org>
Date:	Tue, 20 Aug 2013 16:04:18 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Cc:	Dave Hansen <dave.hansen@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jiri Kosina <jkosina@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] mm/pgtable: Fix continue to preallocate pmds
 even if failure occurrence

On Tue, 20 Aug 2013 14:54:53 +0800 Wanpeng Li <liwanp@...ux.vnet.ibm.com> wrote:

> preallocate_pmds will continue to preallocate pmds even if failure
> occurrence, and then free all the preallocate pmds if there is
> failure, this patch fix it by stop preallocate if failure occurrence
> and go to free path.
>
> ...
>
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -196,21 +196,18 @@ static void free_pmds(pmd_t *pmds[])
>  static int preallocate_pmds(pmd_t *pmds[])
>  {
>  	int i;
> -	bool failed = false;
>  
>  	for(i = 0; i < PREALLOCATED_PMDS; i++) {
>  		pmd_t *pmd = (pmd_t *)__get_free_page(PGALLOC_GFP);
>  		if (pmd == NULL)
> -			failed = true;
> +			goto err;
>  		pmds[i] = pmd;
>  	}
>  
> -	if (failed) {
> -		free_pmds(pmds);
> -		return -ENOMEM;
> -	}
> -
>  	return 0;
> +err:
> +	free_pmds(pmds);
> +	return -ENOMEM;
>  }

Nope.  If the error path is taken, free_pmds() will free uninitialised
items from pmds[], which is a local in pgd_alloc() and contains random
stack junk.  The kernel will crash.

You could pass an nr_pmds argument to free_pmds(), or zero out the
remaining items on the error path.  However, although the current code
is a bit kooky, I don't see that it is harmful in any way.

> Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>

Ahem.
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