Re: We found a bug in i40e_debugfs.c for the latest linux

[Simon Horman <horms@kernel.org>]

On Thu, Jul 10, 2025 at 10:14:18AM +0800, Wang Haoran wrote:
> Hi, my name is Wang Haoran. We found a bug in the
> i40e_dbg_command_read function located in
> drivers/net/ethernet/intel/i40e/i40e_debugfs.c in the latest Linux
> kernel (version 6.15.5).
> The buffer "i40e_dbg_command_buf" has a size of 256. When formatted
> together with the network device name (name), a newline character, and
> a null terminator, the total formatted string length may exceed the
> buffer size of 256 bytes.
> Since "snprintf" returns the total number of bytes that would have
> been written (the length of  "%s: %s\n" ), this value may exceed the
> buffer length passed to copy_to_user(), this will ultimatly cause
> function "copy_to_user" report a buffer overflow error.
> Replacing snprintf with scnprintf ensures the return value never
> exceeds the specified buffer size, preventing such issues.

Thanks Wang Haoran.

I agree that using scnprintf() is a better choice here than snprintf().

But it is not clear to me that this is a bug.

I see that i40e_dbg_command_buf is initialised to be the
empty string. And I don't see it's contents being updated.

While ->name should be no longer than IFNAMSIZ - 1 (=15) bytes long,
excluding the trailing '\0'.

If so, the string formatted by the line below should always
comfortably fit within buf_size (256 bytes).

> 
> --- i40e_debugfs.c 2025-07-06 17:04:26.000000000 +0800
> +++ i40e_debugfs.c 2025-07-09 15:51:47.259130500 +0800
> @@ -70,7 +70,7 @@
>   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);
> 
> Best regards,
> Wang Haoran
>