Subject: Re: [PATCH v2] netdevsim: Add trailing zero to terminate the string in nsim_nexthop_bucket_activity_write()

From: Simon Horman <[email-masked]>
Date: Thu, 24 Oct 2024 11:55:08 +0100

On Tue, Oct 22, 2024 at 12:19:08PM -0500, Gax-c wrote:
> From: Zichen Xie <[email-masked]>
> 
> This was found by a static analyzer.
> We should not forget the trailing zero after copy_from_user()
> if we will further do some string operations, sscanf() in this
> case. Adding a trailing zero will ensure that the function
> performs properly.
> 
> Fixes: c6385c0b67c5 ("netdevsim: Allow reporting activity on nexthop buckets")
> Signed-off-by: Zichen Xie <[email-masked]>
> ---
> v2: adjust code format.
> ---
>  drivers/net/netdevsim/fib.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/netdevsim/fib.c b/drivers/net/netdevsim/fib.c
> index 41e80f78b316..16c382c42227 100644
> --- a/drivers/net/netdevsim/fib.c
> +++ b/drivers/net/netdevsim/fib.c
> @@ -1377,10 +1377,12 @@ static ssize_t nsim_nexthop_bucket_activity_write(struct file *file,
>  
>  	if (pos != 0)
>  		return -EINVAL;
> -	if (size > sizeof(buf))
> +	if (size > sizeof(buf) - 1)

I don't think this change for the best.
If the input data is well formatted it will end with a '\0'.
Which may be copied into the last byte of buf.

With this change the maximum size of the input data is
unnecessarily reduced by one.

>  		return -EINVAL;
>  	if (copy_from_user(buf, user_buf, size))
>  		return -EFAULT;
> +	buf[size] = 0;
> +
>  	if (sscanf(buf, "%u %hu", &nhid, &bucket_index) != 2)
>  		return -EINVAL;
>  
> -- 
> 2.34.1
> 
>