[1/2] USB: yurex: Fix buffer over-read in yurex_write()

[1/2] USB: yurex: Fix buffer over-read in yurex_write()

[Ben Hutchings]

On Thu, 2018-08-16 at 04:15 +0200, Jann Horn wrote:
> On Wed, Aug 15, 2018 at 10:44 PM Ben Hutchings
> <ben.hutchings@codethink.co.uk> wrote:
[...]
> > @@ -446,6 +446,7 @@ static ssize_t yurex_write(struct file *file, const char __user *user_buffer,
> >                 retval = -EFAULT;
> >                 goto error;
> >         }
> > +       buffer[count] = 0;
> >         memset(dev->cntl_buffer, CMD_PADDING, YUREX_BUF_SIZE);
> > 
> >         switch (buffer[0]) {
> 
> By the way: A little bit below here, there's some other line that looks bogus:
> 
> > buffer[6] = CMD_EOF;
> 
> AFAICS that should probably go into ->cntl_buffer; `buffer` and `data`
> aren't used below that point. But that'd just be a functional bug, not
> security-relevant, so I'm not sure whether anyone cares.

Yes I noticed that too.  Since I can't test with the actual hardware, I
left it alone.  For all I know, the firmware actually expects
CMD_PADDING and not CMD_EOF at the end of this command.

Ben.

[1/2] USB: yurex: Fix buffer over-read in yurex_write()

[Jann Horn]

On Wed, Aug 15, 2018 at 10:44 PM Ben Hutchings
<ben.hutchings@codethink.co.uk> wrote:
>
> If the written data starts with a digit, yurex_write() tries to parse
> it as an integer using simple_strtoull().  This requires a null-
> terminator, and currently there's no guarantee that there is one.

Oh, good catch.

> (The sample program at
> https://github.com/NeoCat/YUREX-driver-for-Linux/blob/master/sample/yurex_clock.pl
> writes an integer without a null terminator.  It seems like it must
> have worked by chance!)
>
> Always add a null byte after the written data.  Enlarge the buffer
> to allow for this.
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
> ---
>  drivers/usb/misc/yurex.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/usb/misc/yurex.c b/drivers/usb/misc/yurex.c
> index 3be40eaa1ac9..1232dd49556d 100644
> --- a/drivers/usb/misc/yurex.c
> +++ b/drivers/usb/misc/yurex.c
> @@ -421,13 +421,13 @@ static ssize_t yurex_write(struct file *file, const char __user *user_buffer,
>  {
>         struct usb_yurex *dev;
>         int i, set = 0, retval = 0;
> -       char buffer[16];
> +       char buffer[16 + 1];
>         char *data = buffer;
>         unsigned long long c, c2 = 0;
>         signed long timeout = 0;
>         DEFINE_WAIT(wait);
>
> -       count = min(sizeof(buffer), count);
> +       count = min(sizeof(buffer) - 1, count);
>         dev = file->private_data;
>
>         /* verify that we actually have some data to write */
> @@ -446,6 +446,7 @@ static ssize_t yurex_write(struct file *file, const char __user *user_buffer,
>                 retval = -EFAULT;
>                 goto error;
>         }
> +       buffer[count] = 0;
>         memset(dev->cntl_buffer, CMD_PADDING, YUREX_BUF_SIZE);
>
>         switch (buffer[0]) {

By the way: A little bit below here, there's some other line that looks bogus:

> buffer[6] = CMD_EOF;

AFAICS that should probably go into ->cntl_buffer; `buffer` and `data`
aren't used below that point. But that'd just be a functional bug, not
security-relevant, so I'm not sure whether anyone cares.

[1/2] USB: yurex: Fix buffer over-read in yurex_write()

[Ben Hutchings]

If the written data starts with a digit, yurex_write() tries to parse
it as an integer using simple_strtoull().  This requires a null-
terminator, and currently there's no guarantee that there is one.

(The sample program at
https://github.com/NeoCat/YUREX-driver-for-Linux/blob/master/sample/yurex_clock.pl
writes an integer without a null terminator.  It seems like it must
have worked by chance!)

Always add a null byte after the written data.  Enlarge the buffer
to allow for this.

Cc: stable@vger.kernel.org
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
---
 drivers/usb/misc/yurex.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/misc/yurex.c b/drivers/usb/misc/yurex.c
index 3be40eaa1ac9..1232dd49556d 100644
--- a/drivers/usb/misc/yurex.c
+++ b/drivers/usb/misc/yurex.c
@@ -421,13 +421,13 @@ static ssize_t yurex_write(struct file *file, const char __user *user_buffer,
 {
 	struct usb_yurex *dev;
 	int i, set = 0, retval = 0;
-	char buffer[16];
+	char buffer[16 + 1];
 	char *data = buffer;
 	unsigned long long c, c2 = 0;
 	signed long timeout = 0;
 	DEFINE_WAIT(wait);
 
-	count = min(sizeof(buffer), count);
+	count = min(sizeof(buffer) - 1, count);
 	dev = file->private_data;
 
 	/* verify that we actually have some data to write */
@@ -446,6 +446,7 @@ static ssize_t yurex_write(struct file *file, const char __user *user_buffer,
 		retval = -EFAULT;
 		goto error;
 	}
+	buffer[count] = 0;
 	memset(dev->cntl_buffer, CMD_PADDING, YUREX_BUF_SIZE);
 
 	switch (buffer[0]) {