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Subject: Re: [PATCH] nl80211: prefer struct_size over open coded arithmetic
To:     Len Baker <len.baker@gmx.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>
Cc:     Kees Cook <keescook@chromium.org>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
        linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20210919114040.41522-1-len.baker@gmx.com>
From:   "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Message-ID: <47961463-5b2c-dd6f-0e98-ea95c13409fb@embeddedor.com>
Date:   Tue, 21 Sep 2021 13:51:58 -0500
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On 9/19/21 06:40, Len Baker wrote:
> As noted in the "Deprecated Interfaces, Language Features, Attributes,
> and Conventions" documentation [1], size calculations (especially
> multiplication) should not be performed in memory allocator (or similar)
> function arguments due to the risk of them overflowing. This could lead
> to values wrapping around and a smaller allocation being made than the
> caller was expecting. Using those allocations could lead to linear
> overflows of heap memory and other misbehaviors.
> 
> So, use the struct_size() helper to do the arithmetic instead of the
> argument "size + count * size" in the kzalloc() functions.
> 
> Also, take the opportunity to refactor the memcpy() call to use the
> flex_array_size() helper.
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
> 
> Signed-off-by: Len Baker <len.baker@gmx.com>
> ---
>  net/wireless/nl80211.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/net/wireless/nl80211.c b/net/wireless/nl80211.c
> index bf7cd4752547..b56856349ced 100644
> --- a/net/wireless/nl80211.c
> +++ b/net/wireless/nl80211.c
> @@ -11766,9 +11766,10 @@ static int nl80211_set_cqm_rssi(struct genl_info *info,
>  	wdev_lock(wdev);
>  	if (n_thresholds) {
>  		struct cfg80211_cqm_config *cqm_config;
> +		size_t size = struct_size(cqm_config, rssi_thresholds,
> +					  n_thresholds);
> 
> -		cqm_config = kzalloc(sizeof(struct cfg80211_cqm_config) +
> -				     n_thresholds * sizeof(s32), GFP_KERNEL);
> +		cqm_config = kzalloc(size, GFP_KERNEL);

I don't think variable _size_ is needed here; this is just fine:

-               cqm_config = kzalloc(sizeof(struct cfg80211_cqm_config) +
-                                    n_thresholds * sizeof(s32), GFP_KERNEL);
+               cqm_config = kzalloc(struct_size(cqm_config, rssi_thresholds,
+                                                n_thresholds), GFP_KERNEL);

Thanks
--
Gustavo

>  		if (!cqm_config) {
>  			err = -ENOMEM;
>  			goto unlock;
> @@ -11777,7 +11778,8 @@ static int nl80211_set_cqm_rssi(struct genl_info *info,
>  		cqm_config->rssi_hyst = hysteresis;
>  		cqm_config->n_rssi_thresholds = n_thresholds;
>  		memcpy(cqm_config->rssi_thresholds, thresholds,
> -		       n_thresholds * sizeof(s32));
> +		       flex_array_size(cqm_config, rssi_thresholds,
> +				       n_thresholds));
> 
>  		wdev->cqm_config = cqm_config;
>  	}
> @@ -15081,9 +15083,7 @@ static int nl80211_set_sar_specs(struct sk_buff *skb, struct genl_info *info)
>  	if (specs > rdev->wiphy.sar_capa->num_freq_ranges)
>  		return -EINVAL;
> 
> -	sar_spec = kzalloc(sizeof(*sar_spec) +
> -			   specs * sizeof(struct cfg80211_sar_sub_specs),
> -			   GFP_KERNEL);
> +	sar_spec = kzalloc(struct_size(sar_spec, sub_specs, specs), GFP_KERNEL);
>  	if (!sar_spec)
>  		return -ENOMEM;
> 
> --
> 2.25.1
>