Re: [PATCH V3] USB: usbtmc: prevent kernel-usb-infoleak

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Sep 08, 2024 at 10:20:57AM +0800, Edward Adam Davis wrote:
> The syzbot reported a kernel-usb-infoleak in usbtmc_write.
> 
> The expression "aligned = (transfersize + (USBTMC_HEADER_SIZE + 3)) & ~3;"
> in usbtmcw_write() follows the following pattern:
> 
> aligned = (1 + 12 + 3) & ~3 = 16   // 3 bytes have not been initialized
> aligned = (2 + 12 + 3) & ~3 = 16   // 2 bytes have not been initialized
> aligned = (3 + 12 + 3) & ~3 = 16   // 1 byte has not been initialized
> aligned = (4 + 12 + 3) & ~3 = 16   // All bytes have been initialized
> aligned = (5 + 12 + 3) & ~3 = 20   // 3 bytes have not been initialized
> aligned = (6 + 12 + 3) & ~3 = 20   // 2 bytes have not been initialized
> aligned = (7 + 12 + 3) & ~3 = 20   // 1 byte has not been initialized
> aligned = (8 + 12 + 3) & ~3 = 20   // All bytes have been initialized
> aligned = (9 + 12 + 3) & ~3 = 24
> ...
> 
> Note: #define USBTMC_HEADER_SIZE      12
> 
> This results in the buffer[USBTMC_SEAD_SIZE+transfersize] and its
> subsequent memory not being initialized.
> 
> Fixes: 4ddc645f40e9 ("usb: usbtmc: Add ioctl for vendor specific write")
> Reported-and-tested-by: syzbot+9d34f80f841e948c3fdb@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=9d34f80f841e948c3fdb
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> ---
> V2 -> V3: Update condition and comments
> 
>  drivers/usb/class/usbtmc.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/usb/class/usbtmc.c b/drivers/usb/class/usbtmc.c
> index 6bd9fe565385..faf8c5508997 100644
> --- a/drivers/usb/class/usbtmc.c
> +++ b/drivers/usb/class/usbtmc.c
> @@ -1591,6 +1591,10 @@ static ssize_t usbtmc_write(struct file *filp, const char __user *buf,
>  		goto exit;
>  	}
>  
> +	if (USBTMC_HEADER_SIZE + transfersize < aligned)
> +		memset(&buffer[USBTMC_HEADER_SIZE + transfersize], 0,
> +			aligned - USBTMC_HEADER_SIZE - transfersize);

As this is now a pain to read/understand, and there's no comment
describing it so we'll not really understand it in a few months, let
alone years, how about we just do the trivial thing and make the
allocation with kzalloc() to start with?  And put a comment there saying
why it's zeroed out.

Sorry, I thought this was going to be a lot simpler based on your first
patch than this type of logic.

thanks,

greg k-h