linux-kernel - Re: [Line6linux-devel] [PATCH] line6: Use kmemdup rather than duplicating its implementation

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Message-ID: <20121204222529.GK6568@mwanda>
Date:	Wed, 5 Dec 2012 01:25:29 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Stefan Hajnoczi <stefanha@...il.com>
Cc:	Laurent Navet <laurent.navet@...il.com>,
	devel@...verdev.osuosl.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"line6linux-devel@...ts.sourceforge.net" 
	<line6linux-devel@...ts.sourceforge.net>,
	kernel-janitors@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Line6linux-devel] [PATCH] line6: Use kmemdup rather than
 duplicating its implementation

On Mon, Dec 03, 2012 at 05:34:07PM +0100, Stefan Hajnoczi wrote:
> On Mon, Dec 3, 2012 at 2:20 PM, Laurent Navet <laurent.navet@...il.com> wrote:
> > staging: line6: driver.c
> >  The semantic patch that makes this output is available
> >  in scripts/coccinelle/api/memdup.cocci.
> >
> > Signed-off-by: Laurent Navet <laurent.navet@...il.com>
> > ---
> >  drivers/staging/line6/driver.c |    5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/staging/line6/driver.c b/drivers/staging/line6/driver.c
> > index f5c19b2..e1d6241 100644
> > --- a/drivers/staging/line6/driver.c
> > +++ b/drivers/staging/line6/driver.c
> > @@ -331,14 +331,13 @@ int line6_version_request_async(struct usb_line6 *line6)
> >         char *buffer;
> >         int retval;
> >
> > -       buffer = kmalloc(sizeof(line6_request_version), GFP_ATOMIC);
> > +       buffer = kmemdup(line6_request_version,
> > +                       sizeof(line6_request_version), GFP_ATOMIC);
> >         if (buffer == NULL) {
> >                 dev_err(line6->ifcdev, "Out of memory");
> >                 return -ENOMEM;
> >         }
> >
> > -       memcpy(buffer, line6_request_version, sizeof(line6_request_version));
> > -
> >         retval = line6_send_raw_message_async(line6, buffer,
> >                                               sizeof(line6_request_version));
> >         kfree(buffer);
> > --
> > 1.7.10.4
> 
> Your change is fine but I'm not sure whether we should allocate memory
> in the first place:
> 
> line6_send_raw_message_async() returns before the transfer is
> complete.  It submits one or more URBs but I cannot see a guarantee
> that the buffer is no longer needed.  It seems unsafe to kfree(buffer)
> before the request is complete.
> 

As Greg pointed out we do need to allocate the memory to make DMA
work.  But you're right that it is a use after free bug.  We should
move the kfree(msg->buffer) to inside line6_async_request_sent().
I can send a fix for this tomorrow or if someone else wants to do it
while I'm sleeping that's fine too.  :)

regards,
dan carpenter

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