Re: [PATCH] tty: rfcomm: prefer struct_size over open coded arithmetic

[Kees Cook]

On Sun, Apr 28, 2024 at 03:29:34PM +0200, Erick Archer wrote:
> This is an effort to get rid of all multiplications from allocation
> functions in order to prevent integer overflows [1][2].
> 
> As the "dl" variable is a pointer to "struct rfcomm_dev_list_req" and
> this structure ends in a flexible array:
> 
> struct rfcomm_dev_list_req {
        u16      dev_num;
> 	struct   rfcomm_dev_info dev_info[];
> };

Similar to before, this should gain __counted_by(), and the logic using
dev_info[] refactored slightly to gain coverage.

> 
> the preferred way in the kernel is to use the struct_size() helper to
> do the arithmetic instead of the calculation "size + count * size" in
> the kzalloc() and copy_to_user() functions.
> 
> At the same time remove the "size" variable as it is no longer needed.
> This way, the code is more readable and safer.
> 
> This code was detected with the help of Coccinelle, and audited and
> modified manually.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
> Link: https://github.com/KSPP/linux/issues/160 [2]
> Signed-off-by: Erick Archer <erick.archer@xxxxxxxxxxx>
> ---
> Hi,
> 
> The Coccinelle script used to detect this code pattern is the following:
> 
> virtual report
> 
> @rule1@
> type t1;
> type t2;
> identifier i0;
> identifier i1;
> identifier i2;
> identifier ALLOC =~ "kmalloc|kzalloc|kmalloc_node|kzalloc_node|vmalloc|vzalloc|kvmalloc|kvzalloc";
> position p1;
> @@
> 
> i0 = sizeof(t1) + sizeof(t2) * i1;
> ...
> i2 = ALLOC@p1(..., i0, ...);
> 
> @script:python depends on report@
> p1 << rule1.p1;
> @@
> 
> msg = "WARNING: verify allocation on line %s" % (p1[0].line)
> coccilib.report.print_report(p1[0],msg)
> 
> Regards,
> Erick
> ---
>  net/bluetooth/rfcomm/tty.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/net/bluetooth/rfcomm/tty.c b/net/bluetooth/rfcomm/tty.c
> index 69c75c041fe1..bdc64c8fb85b 100644
> --- a/net/bluetooth/rfcomm/tty.c
> +++ b/net/bluetooth/rfcomm/tty.c
> @@ -504,7 +504,7 @@ static int rfcomm_get_dev_list(void __user *arg)
>  	struct rfcomm_dev *dev;
>  	struct rfcomm_dev_list_req *dl;
>  	struct rfcomm_dev_info *di;
> -	int n = 0, size, err;
> +	int n = 0, err;
>  	u16 dev_num;
>  
>  	BT_DBG("");
> @@ -515,9 +515,7 @@ static int rfcomm_get_dev_list(void __user *arg)
>  	if (!dev_num || dev_num > (PAGE_SIZE * 4) / sizeof(*di))
>  		return -EINVAL;
>  
> -	size = sizeof(*dl) + dev_num * sizeof(*di);

Luckily, "size" can't overflow. Max value seems to be around 1834980,
but I'd rather this be in struct_size() as you have it below. Good!

> -
> -	dl = kzalloc(size, GFP_KERNEL);
> +	dl = kzalloc(struct_size(dl, dev_info, dev_num), GFP_KERNEL);
>  	if (!dl)
>  		return -ENOMEM;

When you add __counted_by, this will need to be added here:

	dl->dev_num = dev_num;


Continuing...

        di = dl->dev_info;
	...
        list_for_each_entry(dev, &rfcomm_dev_list, list) {
                if (!tty_port_get(&dev->port))
                        continue;
                (di + n)->id      = dev->id;
                (di + n)->flags   = dev->flags;
                (di + n)->state   = dev->dlc->state;
                (di + n)->channel = dev->channel;
                bacpy(&(di + n)->src, &dev->src);
                bacpy(&(di + n)->dst, &dev->dst);
                tty_port_put(&dev->port);
                if (++n >= dev_num)
                        break;
        }

Hmpf. I'd rather this code use di[n] instead of (di + n) -- that's much
more idiomatic.

> @@ -542,9 +540,7 @@ static int rfcomm_get_dev_list(void __user *arg)
>  	mutex_unlock(&rfcomm_dev_lock);
>  
>  	dl->dev_num = n;

And this reset of dl->dev_num can stay as-is, since it's reducing the
number of valid entries, in can &rfcomm_dev_list is smaller than the
dev_num count userspace offered.

> -	size = sizeof(*dl) + n * sizeof(*di);
> -
> -	err = copy_to_user(arg, dl, size);
> +	err = copy_to_user(arg, dl, struct_size(dl, dev_info, n));
>  	kfree(dl);
>  

Otherwise looks good!

-Kees

-- 
Kees Cook