linux-kernel - Re: [PATCH] mm: cma: Fix potential null dereference on pointer cma

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Date:   Wed, 17 Mar 2021 00:13:31 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Colin King <colin.king@...onical.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-mm@...ck.org>
CC:     <kernel-janitors@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: cma: Fix potential null dereference on pointer cma

On 3/16/21 3:04 AM, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
> 
> At the start of the function there is a null pointer check on cma
> and then branch to error handling label 'out'.  The subsequent call
> to cma_sysfs_fail_pages_count dereferences cma, hence there is a
> potential null pointer deference issue.  Fix this by only calling

As far as I can tell, it's not possible to actually cause that null
failure with the existing kernel code paths.  *Might* be worth mentioning
that here (unless I'm wrong), but either way, looks good, so:

Reviewed-by: John Hubbard <jhubbard@...dia.com>

thanks,
-- 
John Hubbard
NVIDIA

> cma_sysfs_fail_pages_count if cma is not null.
> 
> Addresses-Coverity: ("Dereference after null check")
> Fixes: dc4764f7e9ac ("mm: cma: support sysfs")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>   mm/cma.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/cma.c b/mm/cma.c
> index 6d4dbafdb318..90e27458ddb7 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -512,7 +512,8 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>   		cma_sysfs_alloc_pages_count(cma, count);
>   	} else {
>   		count_vm_event(CMA_ALLOC_FAIL);
> -		cma_sysfs_fail_pages_count(cma, count);
> +		if (cma)
> +			cma_sysfs_fail_pages_count(cma, count);
>   	}
>   
>   	return page;
>