Re: [PATCH] i2c: mux: harden i2c_mux_alloc() against integer overflows

[Kees Cook <keescook@chromium.org>]

On Thu, Sep 15, 2022 at 02:30:58PM +0300, Dan Carpenter wrote:
> A couple years back we went through the kernel an automatically
> converted size calculations to use struct_size() instead.  The
> struct_size() calculation is protected against integer overflows.
> 
> However it does not make sense to use the result from struct_size()
> for additional math operations as that would negate any safeness.
> 
> Fixes: 1f3b69b6b939 ("i2c: mux: Use struct_size() in devm_kzalloc()")
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> ---
>  drivers/i2c/i2c-mux.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/i2c/i2c-mux.c b/drivers/i2c/i2c-mux.c
> index 774507b54b57..313904be5f3b 100644
> --- a/drivers/i2c/i2c-mux.c
> +++ b/drivers/i2c/i2c-mux.c
> @@ -243,9 +243,10 @@ struct i2c_mux_core *i2c_mux_alloc(struct i2c_adapter *parent,
>  				   int (*deselect)(struct i2c_mux_core *, u32))
>  {
>  	struct i2c_mux_core *muxc;
> +	size_t mux_size;
>  
> -	muxc = devm_kzalloc(dev, struct_size(muxc, adapter, max_adapters)
> -			    + sizeof_priv, GFP_KERNEL);
> +	mux_size = struct_size(muxc, adapter, max_adapters);
> +	muxc = devm_kzalloc(dev, size_add(mux_size, sizeof_priv), GFP_KERNEL);
>  	if (!muxc)
>  		return NULL;
>  	if (sizeof_priv)

The new variable makes it more readable, but beyond that, do you see any
reason not to just directly compose the calls?

diff --git a/drivers/i2c/i2c-mux.c b/drivers/i2c/i2c-mux.c
index 774507b54b57..6c481cde6517 100644
--- a/drivers/i2c/i2c-mux.c
+++ b/drivers/i2c/i2c-mux.c
@@ -244,8 +244,10 @@ struct i2c_mux_core *i2c_mux_alloc(struct i2c_adapter *parent,
 {
 	struct i2c_mux_core *muxc;
 
-	muxc = devm_kzalloc(dev, struct_size(muxc, adapter, max_adapters)
-			    + sizeof_priv, GFP_KERNEL);
+	muxc = devm_kzalloc(dev,
+			    size_add(struct_size(muxc, adapter, max_adapters),
+				     sizeof_priv),
+			    GFP_KERNEL);
 	if (!muxc)
 		return NULL;
 	if (sizeof_priv)

> -- 
> 2.35.1
> 

-- 
Kees Cook