Re: [linux-usb-devel] [PATCH] USB Pegasus driver - avoid a potential NULL pointer dereference.

[Petko Manolov <petkan@nucleusys.com>]

On Sun, 29 Jul 2007, Oliver Neukum wrote:

> Am Sonntag 29 Juli 2007 schrieb Jesper Juhl:
> > On 29/07/07, Satyam Sharma <satyam.sharma@gmail.com> wrote:
> > > Hi,
> > >
> > > On 7/29/07, Jesper Juhl <jesper.juhl@gmail.com> wrote:
> > > > Hello,
> > > >
> > > > This patch makes sure we don't dereference a NULL pointer in
> > > > drivers/net/usb/pegasus.c::write_bulk_callback() in the initial
> > > > struct net_device *net = pegasus->net; assignment.
> > > > The existing code checks if 'pegasus' is NULL and bails out if
> > > > it is, so we better not touch that pointer until after that check.
> > > > [...]
> > > > diff --git a/drivers/net/usb/pegasus.c b/drivers/net/usb/pegasus.c
> > > > index a05fd97..04cba6b 100644
> > > > --- a/drivers/net/usb/pegasus.c
> > > > +++ b/drivers/net/usb/pegasus.c
> > > > @@ -768,11 +768,13 @@ done:
> > > >  static void write_bulk_callback(struct urb *urb)
> > > >  {
> > > >         pegasus_t *pegasus = urb->context;
> > > > -       struct net_device *net = pegasus->net;
> > > > +       struct net_device *net;
> > > >
> > > >         if (!pegasus)
> > > >                 return;
> > > >
> > > > +       net = pegasus->net;
> > > > +
> > > >         if (!netif_device_present(net) || !netif_running(net))
> > > >                 return;
> > >
> > > Is it really possible that we're called into this function with
> > > urb->context == NULL? If not, I'd suggest let's just get rid of
> > > the check itself, instead.
> > I'm not sure. I am not very familiar with this code. I just figured
> > that moving the assignment is potentially a little safer and it is
> > certainly no worse than the current code, so that's a safe and
> > potentially benneficial change. Removing the check may be safe but I'm
> > not certain enough to make that call...
>
> pegasus == NULL there would be a kernel bug. Silently ignoring
> it, like the code now wants to do is bad. As the oops has never been
> reported, I figure turning it into an explicit debugging test is overkill,
> so removal seems to be the best option.

In the past urb->context was not guaranteed to be non-null for any 
asynchronous calls.  If this is not the case anymore then it should be 
removed from at least two more locations in the driver.

Attached you'll find the resulting patch.


		Petko

--- drivers/net/usb/pegasus.c.old	2007-07-10 11:39:50.000000000 +0300
+++ drivers/net/usb/pegasus.c	2007-07-30 11:02:10.000000000 +0300
@@ -100,9 +100,6 @@ static void ctrl_callback(struct urb *ur
 {
 	pegasus_t *pegasus = urb->context;
 
-	if (!pegasus)
-		return;
-
 	switch (urb->status) {
 	case 0:
 		if (pegasus->flags & ETH_REGS_CHANGE) {
@@ -609,15 +606,11 @@ static inline struct sk_buff *pull_skb(p
 static void read_bulk_callback(struct urb *urb)
 {
 	pegasus_t *pegasus = urb->context;
-	struct net_device *net;
+	struct net_device *net = pegasus->net;
 	int rx_status, count = urb->actual_length;
 	u8 *buf = urb->transfer_buffer;
 	__u16 pkt_len;
 
-	if (!pegasus)
-		return;
-
-	net = pegasus->net;
 	if (!netif_device_present(net) || !netif_running(net))
 		return;
 
@@ -770,9 +763,6 @@ static void write_bulk_callback(struct u
 	pegasus_t *pegasus = urb->context;
 	struct net_device *net = pegasus->net;
 
-	if (!pegasus)
-		return;
-
 	if (!netif_device_present(net) || !netif_running(net))
 		return;
 
@@ -805,13 +795,9 @@ static void write_bulk_callback(struct u
 static void intr_callback(struct urb *urb)
 {
 	pegasus_t *pegasus = urb->context;
-	struct net_device *net;
+	struct net_device *net = pegasus->net;
 	int status;
 
-	if (!pegasus)
-		return;
-	net = pegasus->net;
-
 	switch (urb->status) {
 	case 0:
 		break;