linux-kernel - Re: [PATCH 1/1] uio: Fix uio

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Date:   Wed, 14 Jun 2017 07:03:49 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Mike Christie <mchristi@...hat.com>
Cc:     manish.rangankar@...ium.com, target-devel@...r.kernel.org,
        nab@...ux-iscsi.org, kys@...rosoft.com, haiyangz@...rosoft.com,
        sthemmin@...rosoft.com, mst@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] uio: Fix uio_device memory leak

On Tue, Jun 13, 2017 at 07:35:51PM -0500, Mike Christie wrote:
> On 06/13/2017 07:16 PM, Mike Christie wrote:
> > On 06/13/2017 09:01 AM, Greg KH wrote:
> >> > On Wed, Jun 07, 2017 at 03:06:44PM -0500, Mike Christie wrote:
> >>> >> It looks like there might be 2 issues with the uio_device allocation, or it
> >>> >> looks like we are leaking the device for possibly a specific type of device
> >>> >> case that I could not find but one of you may know about.
> >>> >>
> >>> >> Issues:
> >>> >> 1. We use devm_kzalloc to allocate the uio_device, but the release
> >>> >> function, devm_kmalloc_release, is just a noop, so the memory is never freed.
> >> > 
> >> > What do you mean by this?  If the release function is a noop, lots of
> >> > memory in the kernel is leaking.  UIO shouldn't have to do anything
> >> > special here, is the devm api somehow broken?
> > Sorry. I misdiagnosed the problem. It's a noop, but we did kfree on the
> > entire devres and its data later.
> > 
> > The problem I was hitting is that memory is not freed until the parent
> > is removed. __uio_register_device does:
> > 
> >         idev = devm_kzalloc(parent, sizeof(*idev), GFP_KERNEL);
> >         if (!idev) {
> >                 return -ENOMEM;
> >         }
> > 
> > so the devres's memory is associated with the parent. Is that intentional?
> > 
> 
> What I meant is that it I can send a patch to just fix up the
> devm_kzalloc use in uio.c, so it gets the device struct for the uio
> device instead of the parent.
> 
> However, it looks like the existing code using the parent prevents a
> crash. If the child is hot unplugged/removed, and uio_unregister_device
> ends up freeing the idev, then later when the userspace application does
> a close on the uio device we would try to access the freed idev in
> uio_release.
> 
> If the devm_kzalloc parent use was meant for that hot unplug case, then
> I can also look into how to fix the drivers too.

Yeah, I don't know why it is tied to the parent, I'll take a patch to
fix that and let's see what breaks :)

thanks,

greg k-h