[PATCH v2] usb: gadget: f_hid: fixed NULL pointer dereference

[PATCH v2] usb: gadget: f_hid: fixed NULL pointer dereference

[Maxim Devaev]

From: Phil Elwell <phil@raspberrypi.com>

Disconnecting and reconnecting the USB cable can lead to crashes
and a variety of kernel log spam.

The problem was found and reproduced on the Raspberry Pi [1]
and the original fix was created in Raspberry's own fork [2].

Signed-off-by: Maxim Devaev <mdevaev@gmail.com>
Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Link: https://github.com/raspberrypi/linux/issues/3870 [1]
Link: https://github.com/raspberrypi/linux/commit/a6e47d5f4efbd2ea6a0b6565cd2f9b7bb217ded5 [2]
---
 drivers/usb/gadget/function/f_hid.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/gadget/function/f_hid.c b/drivers/usb/gadget/function/f_hid.c
index 02683ac07..08e73e812 100644
--- a/drivers/usb/gadget/function/f_hid.c
+++ b/drivers/usb/gadget/function/f_hid.c
@@ -338,6 +338,11 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
 
 	spin_lock_irqsave(&hidg->write_spinlock, flags);
 
+	if (!hidg->req) {
+		spin_unlock_irqrestore(&hidg->write_spinlock, flags);
+		return -ESHUTDOWN;
+	}
+
 #define WRITE_COND (!hidg->write_pending)
 try_again:
 	/* write queue */
@@ -358,8 +363,14 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
 	count  = min_t(unsigned, count, hidg->report_length);
 
 	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
-	status = copy_from_user(req->buf, buffer, count);
 
+	if (!req) {
+		ERROR(hidg->func.config->cdev, "hidg->req is NULL\n");
+		status = -ESHUTDOWN;
+		goto release_write_pending;
+	}
+
+	status = copy_from_user(req->buf, buffer, count);
 	if (status != 0) {
 		ERROR(hidg->func.config->cdev,
 			"copy_from_user error\n");
@@ -387,14 +398,17 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
 
 	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
 
+	if (!hidg->in_ep->enabled) {
+		ERROR(hidg->func.config->cdev, "in_ep is disabled\n");
+		status = -ESHUTDOWN;
+		goto release_write_pending;
+	}
+
 	status = usb_ep_queue(hidg->in_ep, req, GFP_ATOMIC);
-	if (status < 0) {
-		ERROR(hidg->func.config->cdev,
-			"usb_ep_queue error on int endpoint %zd\n", status);
+	if (status < 0)
 		goto release_write_pending;
-	} else {
+	else
 		status = count;
-	}
 
 	return status;
 release_write_pending:
-- 
2.32.0

Re: [PATCH v2] usb: gadget: f_hid: fixed NULL pointer dereference

[Greg KH]

On Fri, Jul 23, 2021 at 06:59:30PM +0300, Maxim Devaev wrote:
> From: Phil Elwell <phil@raspberrypi.com>
> 
> Disconnecting and reconnecting the USB cable can lead to crashes
> and a variety of kernel log spam.
> 
> The problem was found and reproduced on the Raspberry Pi [1]
> and the original fix was created in Raspberry's own fork [2].
> 
> Signed-off-by: Maxim Devaev <mdevaev@gmail.com>
> Signed-off-by: Phil Elwell <phil@raspberrypi.com>
> Link: https://github.com/raspberrypi/linux/issues/3870 [1]
> Link: https://github.com/raspberrypi/linux/commit/a6e47d5f4efbd2ea6a0b6565cd2f9b7bb217ded5 [2]
> ---
>  drivers/usb/gadget/function/f_hid.c | 26 ++++++++++++++++++++------
>  1 file changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_hid.c b/drivers/usb/gadget/function/f_hid.c
> index 02683ac07..08e73e812 100644
> --- a/drivers/usb/gadget/function/f_hid.c
> +++ b/drivers/usb/gadget/function/f_hid.c
> @@ -338,6 +338,11 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
>  
>  	spin_lock_irqsave(&hidg->write_spinlock, flags);
>  
> +	if (!hidg->req) {
> +		spin_unlock_irqrestore(&hidg->write_spinlock, flags);
> +		return -ESHUTDOWN;
> +	}
> +
>  #define WRITE_COND (!hidg->write_pending)
>  try_again:
>  	/* write queue */
> @@ -358,8 +363,14 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
>  	count  = min_t(unsigned, count, hidg->report_length);
>  
>  	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
> -	status = copy_from_user(req->buf, buffer, count);
>  
> +	if (!req) {
> +		ERROR(hidg->func.config->cdev, "hidg->req is NULL\n");
> +		status = -ESHUTDOWN;
> +		goto release_write_pending;
> +	}
> +
> +	status = copy_from_user(req->buf, buffer, count);
>  	if (status != 0) {
>  		ERROR(hidg->func.config->cdev,
>  			"copy_from_user error\n");
> @@ -387,14 +398,17 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
>  
>  	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
>  
> +	if (!hidg->in_ep->enabled) {
> +		ERROR(hidg->func.config->cdev, "in_ep is disabled\n");
> +		status = -ESHUTDOWN;
> +		goto release_write_pending;

What prevents "enabled" from changing right after you test this?  You do
not have the lock held so could it change then?

> +	}
> +
>  	status = usb_ep_queue(hidg->in_ep, req, GFP_ATOMIC);
> -	if (status < 0) {
> -		ERROR(hidg->func.config->cdev,
> -			"usb_ep_queue error on int endpoint %zd\n", status);
> +	if (status < 0)
>  		goto release_write_pending;

No error message if queueing up failed?

thanks,

greg k-h

Re: [PATCH v2] usb: gadget: f_hid: fixed NULL pointer dereference

[Maxim Devaev]

> > +	if (!hidg->in_ep->enabled) {
> > +		ERROR(hidg->func.config->cdev, "in_ep is disabled\n");
> > +		status = -ESHUTDOWN;
> > +		goto release_write_pending;  
> 
> What prevents "enabled" from changing right after you test this?  You do
> not have the lock held so could it change then?

Inside usb_ep_queue() a similar check is used without blocking.
I suppose this is how it should be. Here it's necessary for the error message.

> > +	}
> > +
> >  	status = usb_ep_queue(hidg->in_ep, req, GFP_ATOMIC);
> > -	if (status < 0) {
> > -		ERROR(hidg->func.config->cdev,
> > -			"usb_ep_queue error on int endpoint %zd\n", status);
> > +	if (status < 0)
> >  		goto release_write_pending;  
> 
> No error message if queueing up failed?

Yes. For each write from userspace, this generates one message.
This can lead to a huge spam in the log.