linux-kernel - [PATCH v2 2/2] staging: comedi: vmk80xx: Fix possible double-free of ->usb_rx

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Message-Id: <20190415115230.7288-1-abbotti@mev.co.uk>
Date:   Mon, 15 Apr 2019 12:52:30 +0100
From:   Ian Abbott <abbotti@....co.uk>
To:     devel@...verdev.osuosl.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ian Abbott <abbotti@....co.uk>,
        H Hartley Sweeten <hsweeten@...ionengravers.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/2] 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@....co.uk>
---
v2: Fix commit message because it incorrectly indicated that the call to
    `kfree()` was being removed from `vmk80xx_auto_attach()`, not
    `vmk80xx_alloc_usb_buffers().
---
 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 b035d662390b..65dc6c51037e 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;
 }
-- 
2.20.1