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

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Date:   Tue, 13 Jun 2017 19:35:51 -0500
From:   Mike Christie <mchristi@...hat.com>
To:     Greg KH <gregkh@...uxfoundation.org>
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 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.