Re: [PATCH] i40e: replace snprintf() with scnprintf()

[Tony Nguyen <anthony.l.nguyen@intel.com>]

On 7/22/2025 4:50 AM, Amir Mohammad Jahangirzad wrote:
> In i40e_dbg_command_read(), a 256-byte buffer is allocated and filled
> using snprintf(), then copied to userspace via copy_to_user().
> 
> The issue is that snprintf() returns the number of characters that
> *Would* have been written, not the number that actually fit in the buffer.
> If the combined length of the netdev name and i40e_dbg_command_buf is
> long (e.g. 288 + 3 bytes), snprintf() still returns 291 - even though only
> 256 bytes were written.

Hi Amir,

Thank you for the patch. In practice, this won't overflow [1]. However, 
this code can be improved. If you follow the thread, there's 
conversation of the changes that will be made.

Thanks,
Tony

[1] https://lore.kernel.org/netdev/20250714181032.GS721198@horms.kernel.org/

> This value is passed to copy_to_user(), which may read past the end of
> the buffer and leak kernel memory to userspace.
> 
> Replacing snprintf() with scnprintf() fixes this. It returns the actual
> number of bytes written, ensuring we only copy valid data.
> 
> Signed-off-by: Amir Mohammad Jahangirzad <a.jahangirzad@gmail.com>
> ---
>   drivers/net/ethernet/intel/i40e/i40e_debugfs.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_debugfs.c b/drivers/net/ethernet/intel/i40e/i40e_debugfs.c
> index 6cd9da662ae1..19a78052800f 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_debugfs.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_debugfs.c
> @@ -70,7 +70,7 @@ static ssize_t i40e_dbg_command_read(struct file *filp, char __user *buffer,
>   		return -ENOSPC;
>   
>   	main_vsi = i40e_pf_get_main_vsi(pf);
> -	len = snprintf(buf, buf_size, "%s: %s\n", main_vsi->netdev->name,
> +	len = scnprintf(buf, buf_size, "%s: %s\n", main_vsi->netdev->name,
>   		       i40e_dbg_command_buf);
>   
>   	bytes_not_copied = copy_to_user(buffer, buf, len);