linux-kernel - Re: [PATCH V2] USB: usbtmc: prevent kernel-usb-infoleak

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Message-ID: <25d5853a-7156-4892-a383-4547e9c95472@rowland.harvard.edu>
Date: Fri, 6 Sep 2024 10:28:11 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Edward Adam Davis <eadavis@...com>
Cc: gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org,
	syzbot+9d34f80f841e948c3fdb@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH V2] USB: usbtmc: prevent kernel-usb-infoleak

On Fri, Sep 06, 2024 at 10:11:03PM +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
> ...

What is the purpose of aligned?  Why doesn't the driver simply use
USBTMC_HEADER_SIZE + transfersize instead of rounding it up to a 
multiple of 4?

> Note: #define USBTMC_HEADER_SIZE      12
> 
> This results in the buffer[USBTMC_SEAD_SIZE+transfersize] and its
> subsequent memory not being initialized.
> 
> The condition aligned < buflen is used to avoid out of bounds access to
> the buffer[USBTMC_HEADER_SIZE + transfersize] when "transfersize = 
> buflen - USBTMC_HEADER_SIZE".
> 
> Fixes: 4ddc645f40e9 ("usb: usbtmc: Add ioctl for vendor specific write")
> Reported-and-tested-by: syzbot+9d34f80f841e948c3fdb@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=9d34f80f841e948c3fdb
> Signed-off-by: Edward Adam Davis <eadavis@...com>
> ---
>  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 (aligned < buflen && (transfersize % 4))

Shouldn't this be

	if (USBTMC_HEADER_SIZE + transfersize < aligned)

?

Alan Stern

> +		memset(&buffer[USBTMC_HEADER_SIZE + transfersize], 0,
> +			aligned - USBTMC_HEADER_SIZE - transfersize);
> +
>  	dev_dbg(&data->intf->dev, "%s(size:%u align:%u)\n", __func__,
>  		(unsigned int)transfersize, (unsigned int)aligned);
>  
> -- 
> 2.43.0