Re: [PATCH 1/4] USB: sisusbvga: change the char buffer from char to u8 for sisusb_write_mem_bulk and sisusb_send_bulk_msg

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Jun 26, 2020 at 03:34:14PM -0400, Changming Liu wrote:
> This patch changes the types of char buffer declarations 
> as well as passed-in parameters to u8 for the function 
> sisusb_write_mem_bulk and sisusb_send_bulk_msg to aviod
> any related UB.
> 
> This patch also change the local buf[4] of sisusb_write_mem_bulk
> to u8. This fixed an undefined behavior, since buf can be filled
> with data from user space, thus can be negative given it's signed, 
> and its content is being left-shifted. Left-shifting a negative
> value is undefined behavior. It's fixed by changing the buf from
> char to u8.

In looking at this closer, it doesn't make sense to change the function
parameters here, as everything that deals with the pointer already
handles the change properly.


> 
> Signed-off-by: Changming Liu <charley.ashbringer@gmail.com>
> ---
>  drivers/usb/misc/sisusbvga/sisusb.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/usb/misc/sisusbvga/sisusb.c b/drivers/usb/misc/sisusbvga/sisusb.c
> index fc8a5da..4aa717a 100644
> --- a/drivers/usb/misc/sisusbvga/sisusb.c
> +++ b/drivers/usb/misc/sisusbvga/sisusb.c
> @@ -327,7 +327,7 @@ static int sisusb_bulkin_msg(struct sisusb_usb_data *sisusb,
>   */
>  
>  static int sisusb_send_bulk_msg(struct sisusb_usb_data *sisusb, int ep, int len,
> -		char *kernbuffer, const char __user *userbuffer, int index,
> +		u8 *kernbuffer, const u8 __user *userbuffer, int index,

So the kernbuffer pointer might want to be changed, but in looking at
the code, there's no difference here, it can be left alone.

The userbuffer does not need to be changed at all.

>  static int sisusb_write_mem_bulk(struct sisusb_usb_data *sisusb, u32 addr,
> -		char *kernbuffer, int length, const char __user *userbuffer,
> +		u8 *kernbuffer, int length, const u8 __user *userbuffer,

Same here, these do not need to be changed.

>  		int index, ssize_t *bytes_written)
>  {
>  	struct sisusb_packet packet;
> @@ -761,7 +761,7 @@ static int sisusb_write_mem_bulk(struct sisusb_usb_data *sisusb, u32 addr,
>  	u8   swap8, fromkern = kernbuffer ? 1 : 0;
>  	u16  swap16;
>  	u32  swap32, flag = (length >> 28) & 1;
> -	char buf[4];
> +	u8 buf[4];

That is what should be changed, and in looking at the code path, I think
that's it here.

Sorry for taking you down the wrong path, but I think you should only
change things that actually matter, and the above api changes don't
change anything at all, right?

thanks,

greg k-h