Re: Re: [PATCH] usbnet: dereference after null...

Re: Re: [PATCH] usbnet: dereference after null check in usbnet_start_xmit() and __usbnet_read_cmd()

From: Vivek Kumar Bhagat <hidden>
Date: 2015-08-20 04:43:38

Dear Bjorn,

quoted

This is wrong.  There are usbnet minidrivers depending on info->tx_fixup
being called with a NULL skb.

Also, if dev_hard_start_xmit() ensures that skb can not be NULL in usbnet_start_xmit()
then we should remove below check.
    if (skb)      <--- This check is confusing which says skb can be NULL.
                skb_tx_timestamp(skb); 


Best Regards,
Vivek

------- Original Message -------
Sender : Bjørn Mork[off-list ref]
Date : Aug 19, 2015 17:21 (GMT+05:30)
Title : Re: [PATCH] usbnet: dereference after null check in usbnet_start_xmit() and __usbnet_read_cmd()

Vivek Kumar Bhagat writes:

usbnet_start_xmit() - If info->tx_fixup is not defined by class driver,
NULL check does not happen for skb pointer and leads to NULL dereference.
__usbnet_read_cmd() - if data pointer is passed as NULL, memcpy will
dereference NULL pointer.

That's two completely different issues.  Mixing them in a single patch
is only confusing things.

quoted hunk

Signed-off-by: Vivek Kumar Bhagat 
---
 drivers/net/usb/usbnet.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 3c86b10..ec4d224 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c

@@ -1294,6 +1294,8 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
 
  if (skb)
  skb_tx_timestamp(skb);
+ else
+ goto drop;
 
  // some devices want funky USB-level framing, for
  // win32 driver (usually) and/or hardware quirks


This is wrong.  There are usbnet minidrivers depending on info->tx_fixup
being called with a NULL skb.

quoted hunk

@@ -1906,7 +1908,8 @@ static int __usbnet_read_cmd(struct usbnet *dev, u8 cmd, u8 reqtype,
  buf = kmalloc(size, GFP_KERNEL);
  if (!buf)
  goto out;
- }
+ } else
+ goto out;
 
  err = usb_control_msg(dev->udev, usb_rcvctrlpipe(dev->udev, 0),
        cmd, reqtype, value, index, buf, size,


This is also wrong.  It makes __usbnet_read_cmd() return -ENOMEM if
called with a NULL data pointer.  I don't know if it is used, but it's
perfectly valid to call __usbnet_read_cmd() with data == NULL if
size == 0. No memcpy will happen in this case because usb_control_msg
can only return 0 or an error

Please don't submit any more such patches without proper justification.
You cannot trust that someone will actually take the time to sanity
check your changes.  Patches claiming to fix a NULL dereference should
at least provide an oops.


Bjørn

Re: [PATCH] usbnet: dereference after null check in usbnet_start_xmit() and __usbnet_read_cmd()

From: Bjørn Mork <bjorn@mork.no>
Date: 2015-08-20 07:29:20

Vivek Kumar Bhagat [off-list ref] writes:

Dear Bjorn,

quoted

This is wrong.  There are usbnet minidrivers depending on info->tx_fixup
being called with a NULL skb.

Also, if dev_hard_start_xmit() ensures that skb can not be NULL in usbnet_start_xmit()
then we should remove below check.
    if (skb)      <--- This check is confusing which says skb can be NULL.
                skb_tx_timestamp(skb);


No, that test is there because of the ugly hack in cdc_ncm.  It doesn't
go through dev_hard_start_xmit(), but calls usbnet_start_xmit() directly
with a NULL skb as a signal to itself.  Yes, I told you it was ugly ;)

I do agree that it would be nice to make this go away.  But until that
happens usbnet_start_xmit() has to deal with NULL skbs, forwarding them
to the tx_fixup hook.


Bjørn

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