Re: [PATCH] USB: usbfs: fix mmap dma mismatch

From: Christoph Hellwig <hch@lst.de>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Christoph Hellwig <hch@lst.de>,
	linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
	Hillf Danton <hdanton@sina.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Jeremy Linton <jeremy.linton@arm.com>,
	syzbot+353be47c9ce21b68b7ed@syzkaller.appspotmail.com,
	stable <stable@vger.kernel.org>
Subject: Re: [PATCH] USB: usbfs: fix mmap dma mismatch
Date: Thu, 14 May 2020 16:39:13 +0200	[thread overview]
Message-ID: <20200514143913.GA27798@lst.de> (raw)
In-Reply-To: <20200514120944.GA2005274@kroah.com>

On Thu, May 14, 2020 at 02:09:44PM +0200, Greg Kroah-Hartman wrote:
> > > +		if (dma_mmap_coherent(hcd->self.sysdev, vma, mem, dma_handle,
> > > +				      size)) {
> > > +			dec_usb_memory_use_count(usbm, &usbm->vma_use_count);
> > > +			return -EAGAIN;
> > > +		}
> > 
> > What about a goto label to share the error handling path?
> 
> I thought about that, but that's a bit messier than the duplicated lines
> here :)

Actually the error handling looks weird, we can just use normal unwinding
here with an extra call to usb_free_coherent.  Also -EAGAIN is a strange
error to return in this case, as it is simply incorrect.  I think passing
through the errors from dma_mmap_coherent and remap_pfn_range would make
a lot more sense.

Last but not least I wonder if this is the right place to open code
the localmem and has_dma checks - from a layering POV it should be
a usb_mmap_coherent helper at the same level as usb_alloc_coherent.