[3.13.y-ckt stable] Patch "gadgetfs: use-after-free in ->aio_read()" has been added to staging queue

[Kamal Mostafa]

On Tue, 2015-04-07 at 13:47 +0100, Luis Henriques wrote:
> On Mon, Apr 06, 2015 at 02:58:30PM -0700, Kamal Mostafa wrote:
> > This is a note to let you know that I have just added a patch titled
> > 
> >     gadgetfs: use-after-free in ->aio_read()
> > 

> This backport will add a compilation warning because the 'value'
> variable may be used uninitialized in function ep_aio_rwtail -- see
> below.

Yes, I see the possible uninit condition now.  Strangely I don't get
that warning on this file in my amd64  build (with any version of gcc).
[[ Noting that you mentioned in irc that you only get the warning when
cross-compiling with powerpc-linux-gnu-gcc (4.8.2). ]]

> Probably, the best thing to do is to add a 'value = -ENOMEM;'
> immediately before the goto statment.

Alas, even that's not sufficient.  For 3.13 at least, there's yet
another "goto fail" path which would end up leaking priv->iv too.

Given the mess, I'm just going to drop this patch from 3.13-stable
altogether, which appears to match other the stables < 3.18.

 -Kamal


> 
> > For more information about the 3.13.y-ckt tree, see
> > https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable
> > 
> > Thanks.
> > -Kamal
> > 
> > ------
> > 
> > From 29e97f7298ed490b12f6c27e60b0607b4f06f081 Mon Sep 17 00:00:00 2001
> > From: Al Viro <viro at zeniv.linux.org.uk>
> > Date: Fri, 6 Feb 2015 02:07:45 -0500
> > Subject: gadgetfs: use-after-free in ->aio_read()
> > 
> > commit f01d35a15fa04162a58b95970fc01fa70ec9dacd upstream.
> > 
> > AIO_PREAD requests call ->aio_read() with iovec on caller's stack, so if
> > we are going to access it asynchronously, we'd better get ourselves
> > a copy - the one on kernel stack of aio_run_iocb() won't be there
> > anymore.  function/f_fs.c take care of doing that, legacy/inode.c
> > doesn't...
> > 
> > Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
> > Signed-off-by: Kamal Mostafa <kamal at canonical.com>
> > ---
> >  drivers/usb/gadget/inode.c | 15 ++++++++++++---
> >  1 file changed, 12 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/usb/gadget/inode.c b/drivers/usb/gadget/inode.c
> > index b94c049..ff3e6e6 100644
> > --- a/drivers/usb/gadget/inode.c
> > +++ b/drivers/usb/gadget/inode.c
> > @@ -568,7 +568,6 @@ static ssize_t ep_copy_to_user(struct kiocb_priv *priv)
> >  		if (total == 0)
> >  			break;
> >  	}
> > -
> >  	return len;
> >  }
> > 
> > @@ -587,6 +586,7 @@ static void ep_user_copy_worker(struct work_struct *work)
> >  	aio_complete(iocb, ret, ret);
> > 
> >  	kfree(priv->buf);
> > +	kfree(priv->iv);
> >  	kfree(priv);
> >  }
> > 
> > @@ -607,6 +607,7 @@ static void ep_aio_complete(struct usb_ep *ep, struct usb_request *req)
> >  	 */
> >  	if (priv->iv == NULL || unlikely(req->actual == 0)) {
> >  		kfree(req->buf);
> > +		kfree(priv->iv);
> >  		kfree(priv);
> >  		iocb->private = NULL;
> >  		/* aio_complete() reports bytes-transferred _and_ faults */
> > @@ -642,7 +643,7 @@ ep_aio_rwtail(
> >  	struct usb_request	*req;
> >  	ssize_t			value;
> > 
> > -	priv = kmalloc(sizeof *priv, GFP_KERNEL);
> > +	priv = kzalloc(sizeof *priv, GFP_KERNEL);
> >  	if (!priv) {
> >  		value = -ENOMEM;
> >  fail:
> > @@ -651,7 +652,14 @@ fail:
> >  	}
> >  	iocb->private = priv;
> >  	priv->iocb = iocb;
> > -	priv->iv = iv;
> > +	if (iv) {
> > +		priv->iv = kmemdup(iv, nr_segs * sizeof(struct iovec),
> > +				   GFP_KERNEL);
> > +		if (!priv->iv) {
> > +			kfree(priv);
> > +			goto fail;
> > +		}
> > +	}
> 
> 
> This is where 'value' may be used without being initialized, if
> kmemdup() fails and the goto statement is executed: 'value' is
> returned in the 'fail' label.
> 
> This issue seems to have been present upstream but this code was
> completely refactored by commit 7fe3976e0f3a ("gadget: switch
> ep_io_operations to ->read_iter/->write_iter") and the issue is gone.
> 
> Probably, the best thing to do is to add a 'value = -ENOMEM;'
> immediately before the goto statment.
> 
> Cheers,
> --
> Luís
> 
> >  	priv->nr_segs = nr_segs;
> >  	INIT_WORK(&priv->work, ep_user_copy_worker);
> > 
> > @@ -691,6 +699,7 @@ fail:
> >  	mutex_unlock(&epdata->lock);
> > 
> >  	if (unlikely(value)) {
> > +		kfree(priv->iv);
> >  		kfree(priv);
> >  		put_ep(epdata);
> >  	} else
> > --
> > 1.9.1
> > 
> > 
> > -- 
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> > kernel-team at lists.ubuntu.com
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>