staging: comedi: vmk80xx: Fix possible double-free of ->usb_rx_buf

This change “staging: comedi: vmk80xx: Fix possible double-free of ->usb_rx_buf” in Linux kernel is authored by Ian Abbott <abbotti [at] mev.co.uk> on Mon Apr 15 12:52:30 2019 +0100.

staging: comedi: vmk80xx: Fix possible double-free of ->usb_rx_buf

`vmk80xx_alloc_usb_buffers()` is called from `vmk80xx_auto_attach()` to
allocate RX and TX buffers for USB transfers.  It allocates
`devpriv->usb_rx_buf` followed by `devpriv->usb_tx_buf`.  If the
allocation of `devpriv->usb_tx_buf` fails, it frees
`devpriv->usb_rx_buf`,  leaving the pointer set dangling, and returns an
error.  Later, `vmk80xx_detach()` will be called from the core comedi
module code to clean up.  `vmk80xx_detach()` also frees both
`devpriv->usb_rx_buf` and `devpriv->usb_tx_buf`, but
`devpriv->usb_rx_buf` may have already been freed, leading to a
double-free error.  Fix it by removing the call to
`kfree(devpriv->usb_rx_buf)` from `vmk80xx_alloc_usb_buffers()`, relying
on `vmk80xx_detach()` to free the memory.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit 663d294.

There are 4 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/staging/comedi/drivers/vmk80xx.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/staging/comedi/drivers/vmk80xx.c b/drivers/staging/comedi/drivers/vmk80xx.c
index b035d66..65dc6c5 100644
--- a/drivers/staging/comedi/drivers/vmk80xx.c
+++ b/drivers/staging/comedi/drivers/vmk80xx.c
@@ -682,10 +682,8 @@ static int vmk80xx_alloc_usb_buffers(struct comedi_device *dev)
 
 	size = usb_endpoint_maxp(devpriv->ep_tx);
 	devpriv->usb_tx_buf = kzalloc(size, GFP_KERNEL);
-	if (!devpriv->usb_tx_buf) {
-		kfree(devpriv->usb_rx_buf);
+	if (!devpriv->usb_tx_buf)
 		return -ENOMEM;
-	}
 
 	return 0;
 }

The commit for this change in Linux stable tree is 663d294 (patch).

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